doc/pcre2pattern.adoc

= pcre2pattern(3)
:doctype: manpage
:manname: pcre2pattern
:manpurpose: PCRE2 pattern syntax
:description: Specification of the regular expressions supported by PCRE2
:manvolnum: 3
:manmanual: PCRE2
:mansource: PCRE2 10.50-DEV
:docdate: 25 October 2025
:revdate: 25 October 2025
:toc: right

== PCRE2 regular expression details

The syntax and semantics of the regular expressions that are supported by PCRE2
are described in detail below. There is a quick-reference syntax summary in the
xref:pcre2syntax.adoc[`pcre2syntax`]
page. PCRE2 tries to match Perl syntax and semantics as closely as it can.
PCRE2 also supports some alternative regular expression syntax that does not
conflict with the Perl syntax in order to provide some compatibility with
regular expressions in Python, .NET, and Oniguruma. There are in addition some
options that enable alternative syntax and semantics that are not the same as
in Perl.

Perl's regular expressions are described in its own documentation, and regular
expressions in general are covered in a number of books, some of which have
copious examples. Jeffrey Friedl's "Mastering Regular Expressions", published
by O'Reilly, covers regular expressions in great detail. This description of
PCRE2's regular expressions is intended as reference material.

This document discusses the regular expression patterns that are supported by
PCRE2 when its main matching function, func:pcre2_match(), is used. PCRE2 also
has an alternative matching function, func:pcre2_dfa_match(), which matches
using a different algorithm that is not Perl-compatible. Some of the features
discussed below are not available when DFA matching is used. The advantages and
disadvantages of the alternative function, and how it differs from the normal
function, are discussed in the
xref:pcre2matching.adoc[`pcre2matching`]
page.

== EBCDIC character codes

Most computers use ASCII or Unicode for encoding characters, and PCRE2 assumes
this by default. However, it can be compiled to run in an environment that uses
the EBCDIC code, which is the case for some IBM mainframe operating systems. In
the sections below, character code values are ASCII or Unicode; in an EBCDIC
environment these characters may have different code values, and there are no
code points greater than 255. Differences in behaviour when PCRE2 is running in
an EBCDIC environment are described in the section <<ebcdic-environments>>
below, which you can ignore unless you really are in an EBCDIC environment.

== Special start-of-pattern items

A number of options that can be passed to func:pcre2_compile() can also be set
by special items at the start of a pattern. These are not Perl-compatible, but
are provided to make these options accessible to pattern writers who are not
able to change the program that processes the pattern. Any number of these
items may appear, but they must all be together right at the start of the
pattern string, and the letters must be in upper case.

=== UTF support

In the 8-bit and 16-bit PCRE2 libraries, characters may be coded either as
single code units, or as multiple UTF-8 or UTF-16 code units. UTF-32 can be
specified for the 32-bit library, in which case it constrains the character
values to valid Unicode code points. To process UTF strings, PCRE2 must be
built to include Unicode support (which is the default). When using UTF strings
you must either call the compiling function with one or both of the const:PCRE2_UTF
or const:PCRE2_MATCH_INVALID_UTF options, or the pattern must start with the special
sequence [.regex]`+(*UTF)+`, which is equivalent to setting the relevant const:PCRE2_UTF. How
setting a UTF mode affects pattern matching is mentioned in several places
below. There is also a summary of features in the
xref:pcre2unicode.adoc[`pcre2unicode`]
page.

Some applications that allow their users to supply patterns may wish to
restrict them to non-UTF data for security reasons. If the const:PCRE2_NEVER_UTF
option is passed to func:pcre2_compile(), [.regex]`+(*UTF)+` is not allowed, and its
appearance in a pattern causes an error.

=== Unicode property support

Another special sequence that may appear at the start of a pattern is [.regex]`+(*UCP)+`.
This has the same effect as setting the const:PCRE2_UCP option: it causes sequences
such as [.regex]`+\d+` and [.regex]`+\w+` to use Unicode properties to determine character types,
instead of recognizing only characters with codes less than 256 via a lookup
table. If also causes upper/lower casing operations to use Unicode properties
for characters with code points greater than 127, even when UTF is not set.
These behaviours can be changed within the pattern; see the section
<<internal-option-setting>>
below.

Some applications that allow their users to supply patterns may wish to
restrict them for security reasons. If the const:PCRE2_NEVER_UCP option is passed to
func:pcre2_compile(), [.regex]`+(*UCP)+` is not allowed, and its appearance in a pattern
causes an error.

=== Locking out empty string matching

Starting a pattern with [.regex]`+(*NOTEMPTY)+` or [.regex]`+(*NOTEMPTY_ATSTART)+` has the same effect
as passing the const:PCRE2_NOTEMPTY or const:PCRE2_NOTEMPTY_ATSTART option to whichever
matching function is subsequently called to match the pattern. These options
lock out the matching of empty strings, either entirely, or only at the start
of the subject.

=== Disabling auto-possessification

If a pattern starts with [.regex]`+(*NO_AUTO_POSSESS)+`, it has the same effect as setting
the const:PCRE2_NO_AUTO_POSSESS option, or calling func:pcre2_set_optimize() with
a const:PCRE2_AUTO_POSSESS_OFF directive. This stops PCRE2 from making quantifiers
possessive when what follows cannot match the repeated item. For example, by
default [.regex]`a+b` is treated as [.regex]`a++b`. For more details, see the
xref:pcre2api.adoc[`pcre2api`]
documentation.

=== Disabling start-up optimizations

If a pattern starts with [.regex]`+(*NO_START_OPT)+`, it has the same effect as setting the
const:PCRE2_NO_START_OPTIMIZE option, or calling func:pcre2_set_optimize() with
a const:PCRE2_START_OPTIMIZE_OFF directive. This disables several optimizations for
quickly reaching "no match" results. For more details, see the
xref:pcre2api.adoc[`pcre2api`]
documentation.

=== Disabling automatic anchoring

If a pattern starts with [.regex]`+(*NO_DOTSTAR_ANCHOR)+`, it has the same effect as
setting the const:PCRE2_NO_DOTSTAR_ANCHOR option, or
calling func:pcre2_set_optimize() with a const:PCRE2_DOTSTAR_ANCHOR_OFF directive.
This disables optimizations that apply to patterns whose top-level branches
all start with [.regex]`+.*+` (match any number of arbitrary characters). For more details,
see the
xref:pcre2api.adoc[`pcre2api`]
documentation.

=== Disabling JIT compilation

If a pattern that starts with [.regex]`+(*NO_JIT)+` is successfully compiled, an attempt by
the application to apply the JIT optimization by calling
func:pcre2_jit_compile() is ignored.

=== Setting match resource limits

The func:pcre2_match() function contains a counter that is incremented every
time it goes round its main loop. The caller of func:pcre2_match() can set a
limit on this counter, which therefore limits the amount of computing resource
used for a match. The maximum depth of nested backtracking can also be limited;
this indirectly restricts the amount of heap memory that is used, but there is
also an explicit memory limit that can be set.

These facilities are provided to catch runaway matches that are provoked by
patterns with huge matching trees. A common example is a pattern with nested
unlimited repeats applied to a long string that does not match. When one of
these limits is reached, func:pcre2_match() gives an error return. The limits
can also be set by items at the start of the pattern of the form

[source,regex,subs=normal]
----
(*LIMIT_HEAP=__d__)
(*LIMIT_MATCH=__d__)
(*LIMIT_DEPTH=__d__)
----

where _d_ is any number of decimal digits. However, the value of the setting must
be less than the value set (or defaulted) by the caller of func:pcre2_match()
for it to have any effect. In other words, the pattern writer can lower the
limits set by the programmer, but not raise them. If there is more than one
setting of one of these limits, the lower value is used. The heap limit is
specified in kibibytes (units of 1024 bytes).

Prior to release 10.30, LIMIT_DEPTH was called LIMIT_RECURSION. This name is
still recognized for backwards compatibility.

The heap limit applies only when the func:pcre2_match() or
func:pcre2_dfa_match() interpreters are used for matching. It does not apply
to JIT. The match limit is used (but in a different way) when JIT is being
used, or when func:pcre2_dfa_match() is called, to limit computing resource
usage by those matching functions. The depth limit is ignored by JIT but is
relevant for DFA matching, which uses function recursion for recursions within
the pattern and for lookaround assertions and atomic groups. In this case, the
depth limit controls the depth of such recursion.

[[newline-conventions]]
=== Newline conventions

PCRE2 supports six different conventions for indicating line breaks in
strings: a single CR (carriage return) character, a single LF (linefeed)
character, the two-character sequence CRLF, any of the three preceding, any
Unicode newline sequence, or the NUL character (binary zero). The
xref:pcre2api.adoc[`pcre2api`]
page has
further discussion
about newlines, and shows how to set the newline convention when calling
func:pcre2_compile().

It is also possible to specify a newline convention by starting a pattern
string with one of the following sequences:

[.regex]`(*CR)`::
    Carriage return
[.regex]`(*LF)`::
    Linefeed
[.regex]`+(*CRLF)+`::
    Carriage return, followed by linefeed
[.regex]`(*ANYCRLF)`::
    Any of the three above
[.regex]`(*ANY)`::
    All Unicode newline sequences
[.regex]`(*NUL)`::
    The NUL character (binary zero)

These override the default and the options given to the compiling function. For
example, on a Unix system where LF is the default newline sequence, the pattern

[source,regex]
----
(*CR)a.b
----

changes the convention to CR. That pattern matches [.subject]`a\nb` because LF is no
longer a newline. If more than one of these settings is present, the last one
is used.

The newline convention affects where the circumflex and dollar assertions are
true. It also affects the interpretation of the dot metacharacter when
const:PCRE2_DOTALL is not set, and the behaviour of [.regex]`\N` when not followed by an
opening brace. However, it does not affect what the [.regex]`\R` escape sequence
matches. By default, this is any Unicode newline sequence, for Perl
compatibility. However, this can be changed; see the section
<<specifying-r-matches>> and the
description of [.regex]`\R` in the section <<newline-sequences>>
below. A change of [.regex]`\R` setting can be combined with a change of newline
convention.

[[specifying-r-matches]]
=== Specifying what [.regex]`\R` matches

It is possible to restrict [.regex]`\R` to match only CR, LF, or CRLF (instead of the
complete set of Unicode line endings) by setting the option const:PCRE2_BSR_ANYCRLF
at compile time. This effect can also be achieved by starting a pattern with
[.regex]`+(*BSR_ANYCRLF)+`. For completeness, [.regex]`+(*BSR_UNICODE)+` is also recognized,
corresponding to const:PCRE2_BSR_UNICODE.

== Characters and metacharacters

A regular expression is a pattern that is matched against a subject string from
left to right. Most characters stand for themselves in a pattern, and match the
corresponding characters in the subject. As a trivial example, the pattern

[source,regex]
----
The quick brown fox
----

matches a portion of a subject string that is identical to itself. When
caseless matching is specified (the const:PCRE2_CASELESS option or [.regex]`+(?i)+` within the
pattern), letters are matched independently of case. Note that there are two
ASCII characters, K and S, that, in addition to their lower case ASCII
equivalents, are case-equivalent with Unicode char:U+212A (Kelvin sign) and char:U+017F
(long S) respectively when either const:PCRE2_UTF or const:PCRE2_UCP is set, unless the
const:PCRE2_EXTRA_CASELESS_RESTRICT option is in force (either passed to
func:pcre2_compile() or set by [.regex]`+(*CASELESS_RESTRICT)+` or [.regex]`+(?r)+` within the
pattern). If the const:PCRE2_EXTRA_TURKISH_CASING option is in force (either passed
to func:pcre2_compile() or set by [.regex]`+(*TURKISH_CASING)+` within the pattern), then
the 'i' letters are matched according to Turkish and Azeri languages.

The power of regular expressions comes from the ability to include wild cards,
character classes, alternatives, and repetitions in the pattern. These are
encoded in the pattern by the use of _metacharacters_, which do not stand
for themselves but instead are interpreted in some special way.

There are two different sets of metacharacters: those that are recognized
anywhere in the pattern except within square brackets, and those that are
recognized within square brackets. Outside square brackets, the metacharacters
are as follows:

[.regex]`+\+`    :: general escape character with several uses
[.regex]`+^+`    :: assert start of string (or line, in multiline mode)
[.regex]`+$+`    :: assert end of string (or line, in multiline mode)
[.regex]`+.+`    :: match any character except newline (by default)
[.regex]`+[+`    :: start character class definition
[.regex]`{vbar}` :: start of alternative branch
[.regex]`+(+`    :: start group or control verb
[.regex]`+)+`    :: end group or control verb
[.regex]`+*+`    :: 0 or more quantifier
[.regex]`{plus}` :: 1 or more quantifier; also "possessive quantifier"
[.regex]`+?+`    :: 0 or 1 quantifier; also quantifier minimizer
[.regex]`+{+`    :: potential start of min/max quantifier

Brace characters [.regex]`+{+` and [.regex]`+}+` are also used to enclose data for constructions such
as [.regex]`+\g{2}+` or [.regex]`\k{__name__}`. In almost all uses of braces, space and/or horizontal
tab characters that follow [.regex]`+{+` or precede [.regex]`+}+` are allowed and are ignored. In the
case of quantifiers, they may also appear before or after the comma. The
exception to this is [.regex]`+\u{...}+` which is an ECMAScript compatibility feature
that is recognized only when the const:PCRE2_EXTRA_ALT_BSUX option is set. ECMAScript
does not ignore such white space; it causes the item to be interpreted as
literal.

Part of a pattern that is in square brackets is called a "character class". In
a character class the only metacharacters are:

[.regex]`+\+`:: general escape character
[.regex]`+^+`:: negate the class, but only if the first character
[.regex]`+-+`:: indicates character range
[.regex]`+[+`:: POSIX character class (if followed by POSIX syntax)
[.regex]`+]+`:: terminates the character class

If a pattern is compiled with the const:PCRE2_EXTENDED option, most white space in
the pattern, other than in a character class, within a [.regex]`+\Q...\E+` sequence, or
between a [.regex]`#` outside a character class and the next newline, inclusive, is
ignored. An escaping backslash can be used to include a white space or a #
character as part of the pattern. If the const:PCRE2_EXTENDED_MORE option is set, the
same applies, but in addition unescaped space and horizontal tab characters are
ignored inside a character class. Note: only these two characters are ignored,
not the full set of pattern white space characters that are ignored outside a
character class. Option settings can be changed within a pattern; see the
section <<internal-option-setting>>
below.

The following sections describe the use of each of the metacharacters.

== Backslash

The backslash character has several uses. Firstly, if it is followed by a
character that is not a digit or a letter, it takes away any special meaning
that character may have. This use of backslash as an escape character applies
both inside and outside character classes.

For example, if you want to match a [.subject]`*` character, you must write [.regex]`+\*+` in the
pattern. This escaping action applies whether or not the following character
would otherwise be interpreted as a metacharacter, so it is always safe to
precede a non-alphanumeric with backslash to specify that it stands for itself.
In particular, if you want to match a backslash, you write [.regex]`+\\+`.

Only ASCII digits and letters have any special meaning after a backslash. All
other characters (in particular, those whose code points are greater than 127)
are treated as literals.

If you want to treat all characters in a sequence as literals, you can do so by
putting them between [.regex]`+\Q+` and [.regex]`+\E+`. Note that this includes white space even when
the const:PCRE2_EXTENDED option is set so that most other white space is ignored. The
behaviour is different from Perl in that $ and @ are handled as literals in
[.regex]`+\Q...\E+` sequences in PCRE2, whereas in Perl, $ and @ cause variable
interpolation. Also, Perl does "double-quotish backslash interpolation" on any
backslashes between [.regex]`+\Q+` and [.regex]`+\E+` which, its documentation says, "may lead to
confusing results". PCRE2 treats a backslash between [.regex]`+\Q+` and [.regex]`+\E+` just like any
other character. Note the following examples:

[cols="1,1,2",options="header"]
|===
|Pattern                       | PCRE2 matches         | Perl matches
|[.regex]`+\Qabc$xyz\E+`       | [.subject]`abc$xyz`   | [.subject]`abc` followed by the contents of `$xyz`
|[.regex]`+\Qabc\$xyz\E+`      | [.subject]`abc\$xyz`  | [.subject]`abc\$xyz`
|[.regex]`+\Qabc\E\$\Qxyz\E+`  | [.subject]`abc$xyz`   | [.subject]`abc$xyz`
|[.regex]`+\QA\B\E+`           | [.subject]`A\B`       | [.subject]`A\B`
|[.regex]`+\Q\\E+`             | [.subject]`\`         | [.subject]`\\E`
|===

The [.regex]`+\Q...\E+` sequence is recognized both inside and outside character classes.
An isolated [.regex]`+\E+` that is not preceded by [.regex]`+\Q+` is ignored. If [.regex]`+\Q+` is not followed
by [.regex]`+\E+` later in the pattern, the literal interpretation continues to the end of
the pattern (that is, [.regex]`+\E+` is assumed at the end). If the isolated [.regex]`+\Q+` is inside
a character class, this causes an error, because the character class is then
not terminated by a closing square bracket.

Another difference from Perl is that any appearance of [.regex]`+\Q+` or [.regex]`+\E+` inside what
might otherwise be a quantifier causes PCRE2 not to recognize the sequence as a
quantifier. Perl recognizes a quantifier if (redundantly) either of the numbers
is inside [.regex]`+\Q...\E+`, but not if the separating comma is. When not recognized as
a quantifier a sequence such as [.regex]`+{\Q1\E,2}+` is treated as the literal string
[.subject]`{1,2}`.

[[non-printing-characters]]
=== Non-printing characters

A second use of backslash provides a way of encoding non-printing characters
in patterns in a visible manner. There is no restriction on the appearance of
non-printing characters in a pattern, but when a pattern is being prepared by
text editing, it is often easier to use one of the following escape sequences
instead of the binary character it represents. In an ASCII or Unicode
environment, these escapes are as follows:

[.regex]`\a`                  :: alarm, that is, the BEL character (char:0x07)
[.regex]`\c__x__`             :: "control-__x__", where __x__ is a non-control ASCII character
[.regex]`\e`                  :: escape (char:0x1B)
[.regex]`\f`                  :: form feed (char:0x0C)
[.regex]`\n`                  :: linefeed (char:0x0A)
[.regex]`\r`                  :: carriage return (char:0x0D) (but see below)
[.regex]`\t`                  :: tab (char:0x09)
[.regex]`\0__dd__`            :: character with octal code 0__dd__
[.regex]`{backslash}__ddd__`  :: character with octal code __ddd__, or back reference
[.regex]`\o{__ddd..__}`       :: character with octal code __ddd..__
[.regex]`\x__hh__`            :: character with hex code __hh__
[.regex]`\x{__hhh..__}`       :: character with hex code __hhh..__
[.regex]`\N{U+__hhh..__}`     :: character with Unicode hex code point __hhh..__

A description of how back references work is given
later,
following the discussion of
parenthesized groups.

By default, after [.regex]`+\x+` that is not followed by {, one or two hexadecimal
digits are read (letters can be in upper or lower case). If the character that
follows [.regex]`+\x+` is neither { nor a hexadecimal digit, an error occurs. This is
different from Perl's default behaviour, which generates a NUL character, but
is in line with the behaviour of Perl's 'strict' mode in re.

Any number of hexadecimal digits may appear between [.regex]`+\x{+` and [.regex]`+}+`. If a character
other than a hexadecimal digit appears between [.regex]`+\x{+` and [.regex]`+}+`, or if there is no
terminating [.regex]`}`, an error occurs.

Characters whose code points are less than 256 can be defined by either of the
two syntaxes for [.regex]`\x` or by an octal sequence. There is no difference in the way
they are handled. For example, [.regex]`+\xdc+` is exactly the same as [.regex]`+\x{dc}+` or [.regex]`+\334+`.
However, using the braced versions does make such sequences easier to read.

Support is available for some ECMAScript (JavaScript) escape sequences via
two compile-time options. If const:PCRE2_ALT_BSUX is set, the sequence [.regex]`\x` followed
by [.regex]`{` is not recognized. Only if [.regex]`\x` is followed by two hexadecimal digits is it
recognized as a character escape. Otherwise it is interpreted as a literal [.subject]`x`
character. In this mode, support for code points greater than 256 is provided
by [.regex]`\u`, which must be followed by four hexadecimal digits; otherwise it is
interpreted as a literal [.subject]`u` character.

const:PCRE2_EXTRA_ALT_BSUX has the same effect as const:PCRE2_ALT_BSUX and, in addition,
[.regex]`+\u{hhh..}+` is recognized as the character specified by hexadecimal code point.
There may be any number of hexadecimal digits, but unlike other places that
also use curly brackets, spaces are not allowed and would result in the string
being interpreted as a literal. This syntax is from ECMAScript 6.

The [.regex]`\N{U+__hhh\...__}` escape sequence is recognized only when PCRE2 is operating in
UTF mode. Perl also uses [.regex]`\N{__name__}` to specify characters by Unicode name; PCRE2
does not support this. Note that when [.regex]`+\N+` is not followed by an opening brace
(curly bracket) it has an entirely different meaning, matching any character
that is not a newline.

There are some legacy applications where the escape sequence [.regex]`+\r+` is expected to
match a newline. If the const:PCRE2_EXTRA_ESCAPED_CR_IS_LF option is set, [.regex]`+\r+` in a
pattern is converted to [.regex]`+\n+` so that it matches a LF (linefeed) instead of a CR
(carriage return) character.

An error occurs if [.regex]`+\c+` is not followed by a character whose ASCII code point
is in the range 32 to 126. The precise effect of [.regex]`\c__x__` is as follows: if _x_ is a
lower case letter, it is converted to upper case. Then bit 6 of the character
(char:0x40) is inverted. Thus [.regex]`+\cA+` to [.regex]`+\cZ+` become char:0x01 to char:0x1A (A is 41, Z is
5A), but [.regex]`+\c{+` becomes char:0x3B ({ is 7B), and [.regex]`+\c;+` becomes char:0x7B (; is 3B). If
the code unit following [.regex]`+\c+` has a code point less than 32 or greater than 126,
a compile-time error occurs.

For differences in the way some escapes behave in EBCDIC environments,
see section <<ebcdic-environments>>
below.

=== Octal escapes and back references

The escape [.regex]`+\o+` must be followed by a sequence of octal digits, enclosed in
braces. An error occurs if this is not the case. This escape provides a way of
specifying character code points as octal numbers greater than 0777, and it
also allows octal numbers and backreferences to be unambiguously distinguished.

If braces are not used, after [.regex]`\0` up to two further octal digits are read.
However, if the const:PCRE2_EXTRA_NO_BS0 option is set, at least one more octal digit
must follow [.regex]`\0` (use [.regex]`\00` to generate a NUL character). Make sure you supply
two digits after the initial zero if the pattern character that follows is
itself an octal digit.

Inside a character class, when a backslash is followed by any octal digit, up
to three octal digits are read to generate a code point. Any subsequent digits
stand for themselves. The sequences [.regex]`\8` and [.regex]`\9` are treated as the literal
characters [.subject]`8` and [.subject]`9`.

Outside a character class, Perl's handling of a backslash followed by a digit
other than 0 is complicated by ambiguity, and Perl has changed over time,
causing PCRE2 also to change. From PCRE2 release 10.45 there is an option
called const:PCRE2_EXTRA_PYTHON_OCTAL that causes PCRE2 to use Python's unambiguous
rules. The next two subsections describe the two sets of rules.

For greater clarity and unambiguity, it is best to avoid following backslash by a
digit greater than zero. Instead, use [.regex]`+\o{...}+` or [.regex]`+\x{...}+`
to specify numerical
character code points, and [.regex]`+\g{...}+` to specify backreferences.

=== Perl rules for non-class backslash 1-9

All the digits that follow the backslash are read as a decimal number. If the
number is less than 10, begins with the digit 8 or 9, or if there are at least
that many previous capture groups in the expression, the entire sequence is
taken as a back reference. Otherwise, up to three octal digits are read to form
a character code. For example:

[.regex]`\040`  ::  is another way of writing an ASCII space
[.regex]`\40`   ::  is the same, provided there are fewer than 40 previous capture groups
[.regex]`\7`    ::  is always a backreference
[.regex]`\11`   ::  might be a backreference, or another way of writing a tab
[.regex]`\011`  ::  is always a tab
[.regex]`\0113` ::  is a tab followed by the character "3"
[.regex]`\113`  ::  might be a backreference, otherwise the character with octal code 113
[.regex]`\377`  ::  might be a backreference, otherwise the value 255 (decimal)
[.regex]`\81`   ::  is always a backreference

Note that octal values of 100 or greater that are specified using this syntax
must not be introduced by a leading zero, because no more than three octal
digits are ever read.

=== Python rules for non-class backslash 1-9

If there are at least three octal digits after the backslash, exactly three are
read as an octal code point number, but the value must be no greater than
377, even in modes where higher code point values are supported. Any
subsequent digits stand for themselves. If there are fewer than three octal
digits, the sequence is taken as a decimal back reference. Thus, for example,
[.regex]`\12` is always a back reference, independent of how many captures there are in
the pattern. An error is generated for a reference to a non-existent capturing
group.

=== Constraints on character values

Characters that are specified using octal or hexadecimal numbers are
limited to certain values, as follows:

8-bit non-UTF mode::
    No greater than char:0xff
16-bit non-UTF mode::
    No greater than char:0xffff
32-bit non-UTF mode::
    No greater than char:0xffffffff
All UTF modes::
    No greater than char:0x10ffff and a valid code point

Invalid Unicode code points are all those in the range char:0xd800 to char:0xdfff (the
so-called "surrogate" code points). The check for these can be disabled by the
caller of func:pcre2_compile() by setting the option
const:PCRE2_EXTRA_ALLOW_SURROGATE_ESCAPES. However, this is possible only in UTF-8
and UTF-32 modes, because these values are not representable in UTF-16.

=== Escape sequences in character classes

All the sequences that define a single character value can be used both inside
and outside character classes. In addition, inside a character class, [.regex]`+\b+` is
interpreted as the backspace character (char:0x08).

When not followed by an opening brace, [.regex]`+\N+` is not allowed in a character class.
[.regex]`+\B+`, [.regex]`+\R+`, and [.regex]`+\X+` are not special inside a character class. Like other
unrecognized alphabetic escape sequences, they cause an error. Outside a
character class, these sequences have different meanings.

=== Unsupported escape sequences

In Perl, the sequences [.regex]`\F`, [.regex]`\l`, [.regex]`\L`, [.regex]`\u`, and [.regex]`\U` are recognized by its string
handler and used to modify the case of following characters. By default, PCRE2
does not support these escape sequences in patterns. However, if either of the
const:PCRE2_ALT_BSUX or const:PCRE2_EXTRA_ALT_BSUX options is set, [.regex]`\U` matches a [.subject]`U`
character, and [.regex]`\u` can be used to define a character by code point, as
described above.

=== Absolute and relative backreferences

The sequence [.regex]`+\g+` followed by a signed or unsigned number, optionally enclosed
in braces, is an absolute or relative backreference. A named backreference
can be coded as [.regex]`\g{__name__}`. Backreferences are discussed
later,
following the discussion of
parenthesized groups.

=== Absolute and relative subroutine calls

For compatibility with Oniguruma, the non-Perl syntax [.regex]`+\g+` followed by a name or
a number enclosed either in angle brackets or single quotes, is an alternative
syntax for referencing a capture group as a subroutine. Details are discussed
later.
Note that [.regex]`+\g{...}+` (Perl syntax) and [.regex]`+\g<...>+` (Oniguruma syntax) are _not_
synonymous. The former is a backreference; the latter is a
subroutine
call.

[[generic-character-types]]
=== Generic character types

Another use of backslash is for specifying generic character types:

[.regex]`\d`:: Any decimal digit.
[.regex]`\D`:: Any character that is not a decimal digit.
[.regex]`\h`:: Any horizontal white space character.
[.regex]`\H`:: Any character that is not a horizontal white space character.
[.regex]`\N`:: Any character that is not a newline.
[.regex]`\s`:: Any white space character.
[.regex]`\S`:: Any character that is not a white space character.
[.regex]`\v`:: Any vertical white space character.
[.regex]`\V`:: Any character that is not a vertical white space character.
[.regex]`\w`:: Any "word" character.
[.regex]`\W`:: Any "non-word" character.

The [.regex]`+\N+` escape sequence has the same meaning as
the [.regex]`.` metacharacter
when const:PCRE2_DOTALL is not set, but setting const:PCRE2_DOTALL does not change the
meaning of [.regex]`+\N+`. Note that when [.regex]`+\N+` is followed by an opening brace it has a
different meaning. See the section <<non-printing-characters>>
above for details. Perl also uses [.regex]`\N{__name__}` to specify characters by Unicode
name; PCRE2 does not support this.

Each pair of lower and upper case escape sequences partitions the complete set
of characters into two disjoint sets. Any given character matches one, and only
one, of each pair. The sequences can appear both inside and outside character
classes. They each match one character of the appropriate type. If the current
matching point is at the end of the subject string, all of them fail, because
there is no character to match.

The default [.regex]`+\s+` characters are HT (9), LF (10), VT (11), FF (12), CR (13), and
space (32), which are defined as white space in the "C" locale. This list may
vary if locale-specific matching is taking place. For example, in some locales
the "non-breaking space" character (char:0xA0) is recognized as white space, and in
others the VT character is not.

A "word" character is an underscore or any character that is a letter or digit.
By default, the definition of letters and digits is controlled by PCRE2's
low-valued character tables, and may vary if locale-specific matching is taking
place (see
"Locale support"
in the
xref:pcre2api.adoc[`pcre2api`]
page). For example, in a French locale such as "fr_FR" in Unix-like systems,
or "french" in Windows, some character codes greater than 127 are used for
accented letters, and these are then matched by [.regex]`+\w+`. The use of locales with
Unicode is discouraged.

By default, characters whose code points are greater than 127 never match [.regex]`+\d+`,
[.regex]`+\s+`, or [.regex]`+\w+`, and always match [.regex]`+\D+`, [.regex]`+\S+`, and [.regex]`+\W+`, although this may be different
for characters in the range 128--255 when locale-specific matching is happening.
These escape sequences retain their original meanings from before Unicode
support was available, mainly for efficiency reasons. If the const:PCRE2_UCP option
is set, the behaviour is changed so that Unicode properties are used to
determine character types, as follows:

[.regex]`\d`:: Any character that matches [.regex]`\p{Nd}` (decimal digit).
[.regex]`\s`:: Any character that matches [.regex]`\p{Z}`, [.regex]`\h`, or [.regex]`\v`.
[.regex]`\w`:: Any character that matches [.regex]`\p{L}`, [.regex]`\p{N}`, [.regex]`\p{Mn}`, or [.regex]`\p{Pc}`.

The addition of [.regex]`+\p{Mn}+` (non-spacing mark) and the replacement of an explicit
test for underscore with a test for [.regex]`+\p{Pc}+` (connector punctuation) happened in
PCRE2 release 10.43. This brings PCRE2 into line with Perl.

The upper case escapes match the inverse sets of characters. Note that [.regex]`\d`
matches only decimal digits, whereas [.regex]`\w` matches any Unicode digit, as well as
other character categories. Note also that const:PCRE2_UCP affects [.regex]`\b`, and
[.regex]`\B` because they are defined in terms of [.regex]`\w` and [.regex]`\W`. Matching these sequences
is noticeably slower when const:PCRE2_UCP is set.

The effect of const:PCRE2_UCP on any one of these escape sequences can be negated by
the options const:PCRE2_EXTRA_ASCII_BSD, const:PCRE2_EXTRA_ASCII_BSS, and
const:PCRE2_EXTRA_ASCII_BSW, respectively. These options can be set and reset within
a pattern by means of an internal option setting
(see below).

The sequences [.regex]`+\h+`, [.regex]`+\H+`, [.regex]`+\v+`, and [.regex]`+\V+`, in contrast to the other sequences, which
match only ASCII characters by default, always match a specific list of code
points, whether or not const:PCRE2_UCP is set. The horizontal space characters are:

char:U+0009:: Horizontal tab (HT)
char:U+0020:: Space
char:U+00A0:: Non-break space
char:U+1680:: Ogham space mark
char:U+180E:: Mongolian vowel separator
char:U+2000:: En quad
char:U+2001:: Em quad
char:U+2002:: En space
char:U+2003:: Em space
char:U+2004:: Three-per-em space
char:U+2005:: Four-per-em space
char:U+2006:: Six-per-em space
char:U+2007:: Figure space
char:U+2008:: Punctuation space
char:U+2009:: Thin space
char:U+200A:: Hair space
char:U+202F:: Narrow no-break space
char:U+205F:: Medium mathematical space
char:U+3000:: Ideographic space

The vertical space characters are:

char:U+000A:: Linefeed (LF)
char:U+000B:: Vertical tab (VT)
char:U+000C:: Form feed (FF)
char:U+000D:: Carriage return (CR)
char:U+0085:: Next line (NEL)
char:U+2028:: Line separator
char:U+2029:: Paragraph separator

In 8-bit, non-UTF-8 mode, only the characters with code points less than 256
are relevant.

[[newline-sequences]]
=== Newline sequences

Outside a character class, by default, the escape sequence [.regex]`+\R+` matches any
Unicode newline sequence. In 8-bit non-UTF-8 mode [.regex]`+\R+` is equivalent to the
following:

[source,regex]
----
(?>\r\n|\n|\x0b|\f|\r|\x85)
----

This is an example of an "atomic group", details of which are given
below.
This particular group matches either the two-character sequence CR followed by
LF, or one of the single characters LF (linefeed, char:U+000A), VT (vertical tab,
char:U+000B), FF (form feed, char:U+000C), CR (carriage return, char:U+000D), or NEL (next
line, char:U+0085). Because this is an atomic group, the two-character sequence is
treated as a single unit that cannot be split.

In other modes, two additional characters whose code points are greater than 255
are added: LS (line separator, char:U+2028) and PS (paragraph separator, char:U+2029).
Unicode support is not needed for these characters to be recognized.

It is possible to restrict [.regex]`+\R+` to match only CR, LF, or CRLF (instead of the
complete set of Unicode line endings) by setting the option const:PCRE2_BSR_ANYCRLF
at compile time. (BSR is an abbreviation for "backslash R".) This can be made
the default when PCRE2 is built; if this is the case, the other behaviour can
be requested via the const:PCRE2_BSR_UNICODE option. It is also possible to specify
these settings by starting a pattern string with one of the following
sequences:

[.regex]`+(*BSR_ANYCRLF)+`::
    CR, LF, or CRLF only.
[.regex]`+(*BSR_UNICODE)+`::
    Any Unicode newline sequence.

These override the default and the options given to the compiling function.
Note that these special settings, which are not Perl-compatible, are recognized
only at the very start of a pattern, and that they must be in upper case. If
more than one of them is present, the last one is used. They can be combined
with a change of newline convention; for example, a pattern can start with:

[source,regex]
----
(*ANY)(*BSR_ANYCRLF)
----

They can also be combined with the [.regex]`+(*UTF)+` or [.regex]`+(*UCP)+` special sequences. Inside a
character class, [.regex]`+\R+` is treated as an unrecognized escape sequence, and causes
an error.

=== Unicode character properties

When PCRE2 is built with Unicode support (the default), three additional escape
sequences that match characters with specific properties are available. They
can be used in any mode, though in 8-bit and 16-bit non-UTF modes these
sequences are of course limited to testing characters whose code points are
less than char:U+0100 or char:U+10000, respectively. In 32-bit non-UTF mode, code points
greater than char:0x10ffff (the Unicode limit) may be encountered. These are all
treated as being in the Unknown script and with an unassigned type.

Matching characters by Unicode property is not fast, because PCRE2 has to do a
multistage table lookup in order to find a character's property. That is why
the traditional escape sequences such as [.regex]`+\d+` and [.regex]`+\w+` do not use Unicode
properties in PCRE2 by default, though you can make them do so by setting the
const:PCRE2_UCP option or by starting the pattern with [.regex]`+(*UCP)+`.

The extra escape sequences that provide property support are:

[.regex]`\p{__xx__}`  :: a character with the _xx_ property
[.regex]`\P{__xx__}`  :: a character without the _xx_ property
[.regex]`\X`          :: a Unicode extended grapheme cluster

For compatibility with Perl, negation can be specified by including a
circumflex between the opening brace and the property. For example, [.regex]`+\p{^Lu}+` is
the same as [.regex]`+\P{Lu}+`.

In accordance with Unicode's "loose matching" rules, ASCII white space
characters, hyphens, and underscores are ignored in the properties represented
by _xx_ above. As well as the space character, ASCII white space can be
tab, linefeed, vertical tab, formfeed, or carriage return.

Some properties are specified as a name only; others as a name and a value,
separated by a colon or an equals sign. The names and values consist of ASCII
letters and digits (with one Perl-specific exception, see below). They are not
case sensitive. Note, however, that the escapes themselves, [.regex]`+\p+` and [.regex]`+\P+`,
_are_ case sensitive. There are abbreviations for many names. The following
examples are all equivalent:

[source,regex]
----
\p{bidiclass=al}
\p{BC=al}
\p{ Bidi_Class : AL }
\p{ Bi-di class = Al }
\P{ ^ Bi-di class = Al }
----

There is support for Unicode script names, Unicode general category properties,
"Any", which matches any character (including newline), Bidi_Class, a number of
binary (yes/no) properties, and some special PCRE2 properties (described
below).
Certain other Perl properties such as "InMusicalSymbols" are not supported by
PCRE2. Note that [.regex]`+\P{Any}+` does not match any characters, so always causes a
match failure.

=== Script properties for [.regex]`+\p+` and [.regex]`+\P+`

There are three different syntax forms for matching a script. Each Unicode
character has a basic script and, optionally, a list of other scripts ("Script
Extensions") with which it is commonly used. Using the Adlam script as an
example, [.regex]`+\p{sc:Adlam}+` matches characters whose basic script is Adlam, whereas
[.regex]`+\p{scx:Adlam}+` matches, in addition, characters that have Adlam in their
extensions list. The full names "script" and "script extensions" for the
property types are recognized and, as for all property specifications, an
equals sign is an alternative to the colon. If a script name is given without a
property type, for example, [.regex]`+\p{Adlam}+`, it is treated as [.regex]`+\p{scx:Adlam}+`. Perl
changed to this interpretation at release 5.26 and PCRE2 changed at release
10.40.

Unassigned characters (and in non-UTF 32-bit mode, characters with code points
greater than char:0x10FFFF) are assigned the "Unknown" script. Others that are not
part of an identified script are lumped together as "Common". The current list
of recognized script names and their 4-character abbreviations can be obtained
by running this command:

[source,shell]
----
pcre2test -LS
----

=== The general category property for [.regex]`+\p+` and [.regex]`+\P+`

Each character has exactly one Unicode general category property, specified by
a two-letter abbreviation. If only one letter is specified with [.regex]`\p` or [.regex]`\P`, it
includes all the general category properties that start with that letter. In
this case, in the absence of negation, the curly brackets in the escape
sequence are optional; these two examples have the same effect:

[source,regex]
----
\p{L}
\pL
----

The following general category property codes are supported:

[.regex]`C`:: Other
[.regex]`Cc`:: Control
[.regex]`Cf`:: Format
[.regex]`Cn`:: Unassigned
[.regex]`Co`:: Private use
[.regex]`Cs`:: Surrogate
[.regex]`L`:: Letter
[.regex]`Lc`:: Cased letter
[.regex]`Ll`:: Lower case letter
[.regex]`Lm`:: Modifier letter
[.regex]`Lo`:: Other letter
[.regex]`Lt`:: Title case letter
[.regex]`Lu`:: Upper case letter
[.regex]`M`:: Mark
[.regex]`Mc`:: Spacing mark
[.regex]`Me`:: Enclosing mark
[.regex]`Mn`:: Non-spacing mark
[.regex]`N`:: Number
[.regex]`Nd`:: Decimal number
[.regex]`Nl`:: Letter number
[.regex]`No`:: Other number
[.regex]`P`:: Punctuation
[.regex]`Pc`:: Connector punctuation
[.regex]`Pd`:: Dash punctuation
[.regex]`Pe`:: Close punctuation
[.regex]`Pf`:: Final punctuation
[.regex]`Pi`:: Initial punctuation
[.regex]`Po`:: Other punctuation
[.regex]`Ps`:: Open punctuation
[.regex]`S`:: Symbol
[.regex]`Sc`:: Currency symbol
[.regex]`Sk`:: Modifier symbol
[.regex]`Sm`:: Mathematical symbol
[.regex]`So`:: Other symbol
[.regex]`Z`:: Separator
[.regex]`Zl`:: Line separator
[.regex]`Zp`:: Paragraph separator
[.regex]`Zs`:: Space separator

Perl originally used the name [.regex]`L&` for the [.regex]`Lc` property. This is still supported
by Perl, but discouraged. PCRE2 also still supports it. This property matches
any character that has the [.regex]`Lu`, [.regex]`Ll`, or [.regex]`Lt` property, in other words, any letter
that is not classified as a modifier or "other". From release 10.45 of PCRE2
the properties [.regex]`Lu`, [.regex]`Ll`, and [.regex]`Lt` are all treated as [.regex]`Lc` when case-independent
matching is set by the const:PCRE2_CASELESS option or [.regex]`(?i)` within the pattern. The
other properties are not affected by caseless matching.

The [.regex]`Cs` (Surrogate) property applies only to characters whose code points are in
the range char:U+D800 to char:U+DFFF. These characters are no different to any other
character when PCRE2 is not in UTF mode (using the 16-bit or 32-bit library).
However, they are not valid in Unicode strings and so cannot be tested by PCRE2
in UTF mode, unless UTF validity checking has been turned off (see the
discussion of const:PCRE2_NO_UTF_CHECK in the
xref:pcre2api.adoc[`pcre2api`]
page).

The long synonyms for property names that Perl supports (such as [.regex]`+\p{Letter}+`)
are not supported by PCRE2, nor is it permitted to prefix any of these
properties with "Is".

No character that is in the Unicode table has the Cn (unassigned) property.
Instead, this property is assumed for any code point that is not in the
Unicode table.

=== Binary (yes/no) properties for [.regex]`\p` and [.regex]`\P`

Unicode defines a number of binary properties, that is, properties whose only
values are true or false. You can obtain a list of those that are recognized by
[.regex]`\p` and [.regex]`\P`, along with their abbreviations, by running this command:

[source,shell]
----
pcre2test -LP
----

=== The Bidi_Class property for [.regex]`\p` and [.regex]`\P`

[.regex]`\p{Bidi_Class:__class__}`:: Matches a character with the given class.
[.regex]`\p{BC:__class__}`:: Matches a character with the given class.

The recognized classes are:

[.regex]`AL`:: Arabic letter
[.regex]`AN`:: Arabic number
[.regex]`B`:: Paragraph separator
[.regex]`BN`:: Boundary neutral
[.regex]`CS`:: Common separator
[.regex]`EN`:: European number
[.regex]`ES`:: European separator
[.regex]`ET`:: European terminator
[.regex]`FSI`:: First strong isolate
[.regex]`L`:: Left-to-right
[.regex]`LRE`:: Left-to-right embedding
[.regex]`LRI`:: Left-to-right isolate
[.regex]`LRO`:: Left-to-right override
[.regex]`NSM`:: Non-spacing mark
[.regex]`ON`:: Other neutral
[.regex]`PDF`:: Pop directional format
[.regex]`PDI`:: Pop directional isolate
[.regex]`R`:: Right-to-left
[.regex]`RLE`:: Right-to-left embedding
[.regex]`RLI`:: Right-to-left isolate
[.regex]`RLO`:: Right-to-left override
[.regex]`S`:: Segment separator
[.regex]`WS`:: White space

As in all property specifications, an equals sign may be used instead of a
colon and the class names are case-insensitive. Only the short names listed
above are recognized; PCRE2 does not at present support any long alternatives.

=== Extended grapheme clusters

The [.regex]`\X` escape matches any number of Unicode characters that form an "extended
grapheme cluster", and treats the sequence as an atomic group
(see below).
Unicode supports various kinds of composite character by giving each character
a grapheme breaking property, and having rules that use these properties to
define the boundaries of extended grapheme clusters. The rules are defined in
Unicode Standard Annex 29, "Unicode Text Segmentation". Unicode 11.0.0
abandoned the use of some previous properties that had been used for emojis.
Instead it introduced various emoji-specific properties. PCRE2 uses only the
Extended Pictographic property.

[.regex]`\X` always matches at least one character. Then it decides whether to add
additional characters according to the following rules for ending a cluster:

. End at the end of the subject string.

. Do not end between CR and LF; otherwise end after any control character.

. Do not break Hangul (a Korean script) syllable sequences. Hangul characters
are of five types: L, V, T, LV, and LVT. An L character may be followed by an
L, V, LV, or LVT character; an LV or V character may be followed by a V or T
character; an LVT or T character may be followed only by a T character.

. Do not end before extending characters or spacing marks or the zero-width
joiner (ZWJ) character. Characters with the "mark" property always have the
"extend" grapheme breaking property.

. Do not end after prepend characters.

. Do not end within emoji modifier sequences or emoji ZWJ (zero-width
joiner) sequences. An emoji ZWJ sequence consists of a character with the
Extended_Pictographic property, optionally followed by one or more characters
with the Extend property, followed by the ZWJ character, followed by another
Extended_Pictographic character.

. Do not break within emoji flag sequences. That is, do not break between
regional indicator (RI) characters if there are an odd number of RI characters
before the break point.

. Otherwise, end the cluster.

=== PCRE2's additional properties

As well as the standard Unicode properties described above, PCRE2 supports four
more that make it possible to convert traditional escape sequences such as [.regex]`\w`
and [.regex]`\s` to use Unicode properties. PCRE2 uses these non-standard, non-Perl
properties internally when const:PCRE2_UCP is set. However, they may also be used
explicitly. These properties are:

[.regex]`Xan`:: Any alphanumeric character
[.regex]`Xps`:: Any POSIX space character
[.regex]`Xsp`:: Any Perl space character
[.regex]`Xwd`:: Any Perl "word" character

[.regex]`Xan` matches characters that have either the [.regex]`L` (letter) or the [.regex]`N` (number)
property. [.regex]`Xps` matches the characters tab, linefeed, vertical tab, form feed, or
carriage return, and any other character that has the [.regex]`Z` (separator) property
(this includes the space character). [.regex]`Xsp` is the same as [.regex]`Xps`; in PCRE1 it used
to exclude vertical tab, for Perl compatibility, but Perl changed. [.regex]`Xwd` matches
the same characters as [.regex]`Xan`, plus those that match [.regex]`Mn` (non-spacing mark) or [.regex]`Pc`
(connector punctuation, which includes underscore).

There is another non-standard property, [.regex]`Xuc`, which matches any character that
can be represented by a Universal Character Name in C{plus}{plus} and other programming
languages. These are the characters $, @, ` (grave accent), and all characters
with Unicode code points greater than or equal to char:U+00A0, except for the
surrogates char:U+D800 to char:U+DFFF. Note that most base (ASCII) characters are
excluded. (Universal Character Names are of the form [.regex]`\u__HHHH__` or [.regex]`\U__HHHHHHHH__`
where H is a hexadecimal digit. Note that the [.regex]`Xuc` property does not match these
sequences but the characters that they represent.)

=== Resetting the match start

In normal use, the escape sequence [.regex]`\K` causes any previously matched characters
not to be included in the final matched sequence that is returned. For example,
the pattern:

[source,regex]
----
foo\Kbar
----

matches [.subject]`foobar`, but reports that it has matched [.subject]`bar`. [.regex]`\K` does not interact
with anchoring in any way. The pattern:

[source,regex]
----
^foo\Kbar
----

matches only when the subject begins with [.subject]`foobar` (in single line mode),
though it again reports the matched string as [.subject]`bar`. This feature is similar to
a lookbehind assertion
(described below),
but the part of the pattern that precedes [.regex]`+\K+` is not constrained to match a
limited number of characters, as is required for a lookbehind assertion. The
use of [.regex]`+\K+` does not interfere with the setting of
captured substrings.
For example, when the pattern

[source,regex]
----
(foo)\Kbar
----

matches [.subject]`foobar`, the first substring is still set to [.subject]`foo`.

From version 5.32.0 Perl forbids the use of [.regex]`+\K+` in lookaround assertions. From
release 10.38 PCRE2 also forbids this by default. However, the
const:PCRE2_EXTRA_ALLOW_LOOKAROUND_BSK option can be used when calling
func:pcre2_compile() to re-enable the previous behaviour. When this option is
set, [.regex]`+\K+` is acted upon when it occurs inside positive assertions, but is
ignored in negative assertions. Note that when a pattern such as [.regex]`+(?=ab\K)+`
matches, the reported start of the match can be greater than the end of the
match. Using [.regex]`+\K+` in a lookbehind assertion at the start of a pattern can also
lead to odd effects. For example, consider this pattern:

[source,regex]
----
(?<=\Kfoo)bar
----

If the subject is [.subject]`foobar`, a call to func:pcre2_match() with a starting
offset of 3 succeeds and reports the matching string as [.subject]`foobar`, that is, the
start of the reported match is earlier than where the match started.

=== Simple assertions

The final use of backslash is for certain simple assertions. An assertion
specifies a condition that has to be met at a particular point in a match,
without consuming any characters from the subject string. The use of
groups for more complicated assertions is described
below.
The backslashed assertions are:

[.regex]`\b`:: Matches at a word boundary.
[.regex]`\B`:: Matches when not at a word boundary.
[.regex]`\A`:: Matches at the start of the subject.
[.regex]`\Z`:: Matches at the end of the subject; also matches before a newline at the end of the subject.
[.regex]`\z`:: Matches only at the end of the subject.
[.regex]`\G`:: Matches at the first matching position in the subject.

Inside a character class, [.regex]`+\b+` has a different meaning; it matches the backspace
character. If any other of these assertions appears in a character class, an
"invalid escape sequence" error is generated.

A word boundary is a position in the subject string where the current character
and the previous character do not both match [.regex]`\w` or [.regex]`\W` (i.e. one matches
[.regex]`\w` and the other matches [.regex]`\W`), or the start or end of the string if the
first or last character matches [.regex]`\w`, respectively. When PCRE2 is built with
Unicode support, the meanings of [.regex]`\w` and [.regex]`\W` can be changed by setting the
const:PCRE2_UCP option. When this is done, it also affects [.regex]`\b` and [.regex]`\B`. Neither PCRE2
nor Perl has a separate "start of word" or "end of word" metasequence. However,
whatever follows [.regex]`\b` normally determines which it is. For example, the fragment
[.regex]`+\ba+` matches [.subject]`a` at the start of a word.

The [.regex]`\A`, [.regex]`\Z`, and [.regex]`\z` assertions differ from the traditional circumflex and
dollar (described in the section <<circumflex-and-dollar>>) in that they only ever match at the very
start and end of the subject string, whatever options are set. Thus, they are
independent of multiline mode. These three assertions are not affected by the
const:PCRE2_NOTBOL or const:PCRE2_NOTEOL options, which affect only the behaviour of the
circumflex and dollar metacharacters. However, if the arg:start_offset
argument of func:pcre2_match() is non-zero, indicating that matching is to
start at a point other than the beginning of the subject, [.regex]`\A` can never match.
The difference between [.regex]`\Z` and [.regex]`\z` is that [.regex]`\Z` matches before a newline at the
end of the string as well as at the very end, whereas [.regex]`\z` matches only at the
end.

The [.regex]`\G` assertion is true only when the current matching position is at the
start point of the matching process, as specified by the arg:start_offset
argument of func:pcre2_match(). It differs from [.regex]`\A` when the value of
arg:start_offset is non-zero. By calling func:pcre2_match() multiple times
with appropriate arguments, you can mimic Perl's /g option, and it is in this
kind of implementation where [.regex]`\G` can be useful.

Note, however, that PCRE2's implementation of [.regex]`\G`, being true at the starting
character of the matching process, is subtly different from Perl's, which
defines it as true at the end of the previous match. In Perl, these can be
different when the previously matched string was empty. Because PCRE2 does just
one match at a time, it cannot reproduce this behaviour.

If all the alternatives of a pattern begin with [.regex]`\G`, the expression is anchored
to the starting match position, and the "anchored" flag is set in the compiled
regular expression.

[[circumflex-and-dollar]]
== Circumflex and dollar

The circumflex and dollar metacharacters are zero-width assertions. That is,
they test for a particular condition being true without consuming any
characters from the subject string. These two metacharacters are concerned with
matching the starts and ends of lines. If the newline convention is set so that
only the two-character sequence CRLF is recognized as a newline, isolated CR
and LF characters are treated as ordinary data characters, and are not
recognized as newlines.

Outside a character class, in the default matching mode, the circumflex
character is an assertion that is true only if the current matching point is at
the start of the subject string. If the arg:start_offset argument of
func:pcre2_match() is non-zero, or if const:PCRE2_NOTBOL is set, circumflex can
never match if the const:PCRE2_MULTILINE option is unset. Inside a character class,
circumflex has an entirely different meaning
(see below).

Circumflex need not be the first character of the pattern if a number of
alternatives are involved, but it should be the first thing in each alternative
in which it appears if the pattern is ever to match that branch. If all
possible alternatives start with a circumflex, that is, if the pattern is
constrained to match only at the start of the subject, it is said to be an
"anchored" pattern. (There are also other constructs that can cause a pattern
to be anchored.)

The dollar character is an assertion that is true only if the current matching
point is at the end of the subject string, or immediately before a newline at
the end of the string (by default), unless const:PCRE2_NOTEOL is set. Note, however,
that it does not actually match the newline. Dollar need not be the last
character of the pattern if a number of alternatives are involved, but it
should be the last item in any branch in which it appears. Dollar has no
special meaning in a character class.

The meaning of dollar can be changed so that it matches only at the very end of
the string, by setting the const:PCRE2_DOLLAR_ENDONLY option at compile time. This
does not affect the [.regex]`\Z` assertion.

The meanings of the circumflex and dollar metacharacters are changed if the
const:PCRE2_MULTILINE option is set. When this is the case, a dollar character
matches before any newlines in the string, as well as at the very end, and a
circumflex matches immediately after internal newlines as well as at the start
of the subject string. It does not match after a newline that ends the string,
for compatibility with Perl. However, this can be changed by setting the
const:PCRE2_ALT_CIRCUMFLEX option.

For example, the pattern [.regex]`^abc$` matches the subject string [.subject]`def\nabc` (where
[.regex]`\n` represents a newline) in multiline mode, but not otherwise. Consequently,
patterns that are anchored in single line mode because all branches start with
^ are not anchored in multiline mode, and a match for circumflex is possible
when the arg:start_offset argument of func:pcre2_match() is non-zero. The
const:PCRE2_DOLLAR_ENDONLY option is ignored if const:PCRE2_MULTILINE is set.

When the newline convention (see
"Newline conventions"
below) recognizes the two-character sequence CRLF as a newline, this is
preferred, even if the single characters CR and LF are also recognized as
newlines. For example, if the newline convention is "any", a multiline mode
circumflex matches before [.subject]`xyz` in the string [.subject]`abc\r\nxyz` rather than after
CR, even though CR on its own is a valid newline. (It also matches at the very
start of the string, of course.)

Note that the sequences [.regex]`\A`, [.regex]`\Z`, and [.regex]`\z` can be used to match the start and
end of the subject in both modes, and if all branches of a pattern start with
[.regex]`\A` it is always anchored, whether or not const:PCRE2_MULTILINE is set.

== Full stop (period, dot) and [.regex]`+\n+`

Outside a character class, a dot in the pattern matches any one character in
the subject string except (by default) a character that signifies the end of a
line. One or more characters may be specified as line terminators (see
"Newline conventions"
above).

Dot never matches a single line-ending character. When the two-character
sequence CRLF is the only line ending, dot does not match CR if it is
immediately followed by LF, but otherwise it matches all characters (including
isolated CRs and LFs). When ANYCRLF is selected for line endings, no occurrences
of CR of LF match dot. When all Unicode line endings are being recognized, dot
does not match CR or LF or any of the other line ending characters.

The behaviour of dot with regard to newlines can be changed. If the
const:PCRE2_DOTALL option is set, a dot matches any one character, without exception.
If the two-character sequence CRLF is present in the subject string, it takes
two dots to match it.

The handling of dot is entirely independent of the handling of circumflex and
dollar, the only relationship being that they both involve newlines. Dot has no
special meaning in a character class.

The escape sequence [.regex]`+\N+` when not followed by an opening brace behaves like a
dot, except that it is not affected by the const:PCRE2_DOTALL option. In other words,
it matches any character except one that signifies the end of a line.

When [.regex]`+\N+` is followed by an opening brace it has a different meaning. See the
section <<non-printing-characters>>
above for details. Perl also uses [.regex]`\N{__name__}` to specify characters by Unicode
name; PCRE2 does not support this.

== Matching a single code unit

Outside a character class, the escape sequence [.regex]`\C` matches any one code unit,
whether or not a UTF mode is set. In the 8-bit library, one code unit is one
byte; in the 16-bit library it is a 16-bit unit; in the 32-bit library it is a
32-bit unit. Unlike a dot, [.regex]`\C` always matches line-ending characters. The
feature is provided in Perl in order to match individual bytes in UTF-8 mode,
but it is unclear how it can usefully be used.

Because [.regex]`+\C+` breaks up characters into individual code units, matching one unit
with [.regex]`+\C+` in UTF-8 or UTF-16 mode means that the rest of the string may start
with a malformed UTF character. This has undefined results, because PCRE2
assumes that it is matching character by character in a valid UTF string (by
default it checks the subject string's validity at the start of processing
unless the const:PCRE2_NO_UTF_CHECK or const:PCRE2_MATCH_INVALID_UTF option is used).

An application can lock out the use of [.regex]`+\C+` by setting the
const:PCRE2_NEVER_BACKSLASH_C option when compiling a pattern. It is also possible to
build PCRE2 with the use of [.regex]`+\C+` permanently disabled.

PCRE2 does not allow [.regex]`+\C+` to appear in lookbehind assertions
(described below)
in UTF-8 or UTF-16 modes, because this would make it impossible to calculate
the length of the lookbehind. Neither the alternative matching function
func:pcre2_dfa_match() nor the JIT optimizer support [.regex]`\C` in these UTF modes.
The former gives a match-time error; the latter fails to optimize and so the
match is always run using the interpreter.

In the 32-bit library, however, [.regex]`+\C+` is always supported (when not explicitly
locked out) because it always matches a single code unit, whether or not UTF-32
is specified.

In general, the [.regex]`+\C+` escape sequence is best avoided. However, one way of using
it that avoids the problem of malformed UTF-8 or UTF-16 characters is to use a
lookahead to check the length of the next character, as in this pattern, which
could be used with a UTF-8 string (ignore white space and line breaks):

[source,regex]
----
(?| (?=[\x00-\x7f])(\C) |
    (?=[\x80-\x{7ff}])(\C)(\C) |
    (?=[\x{800}-\x{ffff}])(\C)(\C)(\C) |
    (?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C))
----

In this example, a group that starts with [.regex]`+(?|+` resets the capturing parentheses
numbers in each alternative (see
"Duplicate Group Numbers"
below). The assertions at the start of each branch check the next UTF-8
character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
character's individual bytes are then captured by the appropriate number of
[.regex]`+\C+` groups.

== Square brackets and character classes

An opening square bracket introduces a character class, terminated by a closing
square bracket. A closing square bracket on its own is not special by default.
If a closing square bracket is required as a member of the class, it should be
the first data character in the class (after an initial circumflex, if present)
or escaped with a backslash. This means that, by default, an empty class cannot
be defined. However, if the const:PCRE2_ALLOW_EMPTY_CLASS option is set, a closing
square bracket at the start does end the (empty) class.

A character class matches a single character in the subject. A matched
character must be in the set of characters defined by the class, unless the
first character in the class definition is a circumflex, in which case the
subject character must not be in the set defined by the class. If a circumflex
is actually required as a member of the class, ensure it is not the first
character, or escape it with a backslash.

For example, the character class [.regex]`[aeiou]` matches any lower case English vowel,
whereas [.regex]`[^aeiou]` matches all other characters. Note that a circumflex is just a
convenient notation for specifying the characters that are in the class by
enumerating those that are not. A class that starts with a circumflex is not an
assertion; it still consumes a character from the subject string, and therefore
it fails to match if the current pointer is at the end of the string.

Characters in a class may be specified by their code points using [.regex]`+\o+`, [.regex]`+\x+`, or
[.regex]`\N{U+__hh\...__}` in the usual way. When caseless matching is set, any letters in a
class represent both their upper case and lower case versions, so for example,
a caseless [.regex]`[aeiou]` matches [.subject]`A` as well as [.subject]`a`, and a caseless [.regex]`[^aeiou]` does not
match [.subject]`A`, whereas a caseful version would. Note that there are two ASCII
characters, K and S, that, in addition to their lower case ASCII equivalents,
are case-equivalent with Unicode char:U+212A (Kelvin sign) and char:U+017F (long S)
respectively when either const:PCRE2_UTF or const:PCRE2_UCP is set. If you do not want
these ASCII/non-ASCII case equivalences, you can suppress them by setting
const:PCRE2_EXTRA_CASELESS_RESTRICT, either as an option in a compile context, or by
including [.regex]`+(*CASELESS_RESTRICT)+` or [.regex]`+(?r)+` within a pattern.

Characters that might indicate line breaks are never treated in any special way
when matching character classes, whatever line-ending sequence is in use, and
whatever setting of the const:PCRE2_DOTALL and const:PCRE2_MULTILINE options is used. A
class such as [.regex]`[^a]` always matches one of these characters.

The generic character type escape sequences [.regex]`+\d+`, [.regex]`+\D+`, [.regex]`+\h+`, [.regex]`+\H+`, [.regex]`+\p+`, [.regex]`+\P+`, [.regex]`+\s+`,
[.regex]`+\S+`, [.regex]`+\v+`, [.regex]`+\V+`, [.regex]`+\w+`, and [.regex]`+\W+` may appear in a character class, and add the
characters that they match to the class. For example, [.regex]`[\dABCDEF]` matches any
hexadecimal digit. In UTF modes, the const:PCRE2_UCP option affects the meanings of
[.regex]`+\d+`, [.regex]`+\s+`, [.regex]`+\w+` and their upper case partners, just as it does when they appear
outside a character class, as described in the section
<<generic-character-types>>
above. The escape sequence [.regex]`+\b+` has a different meaning inside a character
class; it matches the backspace character. The sequences [.regex]`+\B+`, [.regex]`+\R+`, and [.regex]`+\X+` are
not special inside a character class. Like any other unrecognized escape
sequences, they cause an error. The same is true for [.regex]`+\N+` when not followed by
an opening brace.

The minus (hyphen) character can be used to specify a range of characters in a
character class. For example, [.regex]`[d-m]` matches any letter between d and m,
inclusive. If a minus character is required in a class, it must be escaped with
a backslash or appear in a position where it cannot be interpreted as
indicating a range, typically as the first or last character in the class,
or immediately after a range. For example, [.regex]`[b-d-z]` matches letters in the range
b to d, a hyphen character, or z.

There is some special treatment for alphabetic ranges in EBCDIC environments;
see the section <<ebcdic-environments>>
below.

Perl treats a hyphen as a literal if it appears before or after a POSIX class
(see below) or before or after a character type escape such as [.regex]`+\d+` or [.regex]`+\H+`.
However, unless the hyphen is the last character in the class, Perl outputs a
warning in its warning mode, as this is most likely a user error. As PCRE2 has
no facility for warning, an error is given in these cases.

It is not possible to have the literal character [.regex]`+]+` as the end character of a
range. A pattern such as [.regex]`[W-]46]` is interpreted as a class of two characters
(`W` and `-`) followed by a literal string [.regex]`46]`, so it would match [.subject]`W46]` or
[.subject]`-46]`. However, if the [.regex]`+]+` is escaped with a backslash it is interpreted as
the end of a range, so [.regex]`[W-\]46]` is interpreted as a class containing a range
and two other characters. The octal or hexadecimal representation of [.regex]`+]+` can
also be used to end a range.

Ranges normally include all code points between the start and end characters,
inclusive. They can also be used for code points specified numerically, for
example [.regex]`[\000-\037]`. Ranges can include any characters that are valid for the
current mode. In any UTF mode, the so-called "surrogate" characters (those
whose code points lie between char:0xd800 and char:0xdfff inclusive) may not be specified
explicitly by default (the const:PCRE2_EXTRA_ALLOW_SURROGATE_ESCAPES option disables
this check). However, ranges such as [.regex]`[\x{d7ff}-\x{e000}]`, which include the
surrogates, are always permitted.

If a range that includes letters is used when caseless matching is set, it
matches the letters in either case. For example, [.regex]`[W-c]` is equivalent to
[.regex]``[][\\^_`wxyzabc]``, matched caselessly, and in a non-UTF mode, if character
tables for a French locale are in use, [.regex]`[\xc8-\xcb]` matches accented E
characters in both cases.

A circumflex can conveniently be used with the upper case character types to
specify a more restricted set of characters than the matching lower case type.
For example, the class [.regex]`[^\W_]` matches any letter or digit, but not underscore,
whereas [.regex]`[\w]` includes underscore. A positive character class should be read as
"something OR something OR ..." and a negative class as "NOT something AND NOT
something AND NOT ...".

The metacharacters that are recognized in character classes are backslash,
hyphen (when it can be interpreted as specifying a range), circumflex
(only at the start), and the terminating closing square bracket. An opening
square bracket is also special when it can be interpreted as introducing a
POSIX class (see
"Posix character classes"
below), or a special compatibility feature (see
"Compatibility feature for word boundaries"
below. Escaping any non-alphanumeric character in a class turns it into a
literal, whether or not it would otherwise be a metacharacter.

== Perl extended character classes

From release 10.45 PCRE2 supports Perl's [.regex]`+(?[...])+` extended character class
syntax. This can be used to perform set operations such as intersection on
character classes.

The syntax permitted within [.regex]`+(?[...])+` is quite different to ordinary character
classes. Inside the extended class, there is an expression syntax consisting of
"atoms", operators, and ordinary parentheses [.regex]`()` used for grouping. Such
classes always have the Perl [.regex]`/xx` modifier (PCRE2 option const:PCRE2_EXTENDED_MORE)
turned on within them. This means that literal space and tab characters are
ignored everywhere in the class.

The allowed atoms are individual characters specified by escape sequences such
as [.regex]`+\n+` or [.regex]`+\x{123}+`, character types such as [.regex]`+\d+`, POSIX classes such as
[.regex]`[:alpha:]`, and nested ordinary (non-extended) character classes. For example,
in [.regex]`+(?[\d & [...]])+` the nested class [.regex]`+[...]+` follows the usual rules for ordinary
character classes, in which parentheses are not metacharacters, and character
literals and ranges are permitted.

Character literals and ranges may not appear outside a nested ordinary
character class because they are not atoms in the extended syntax. The extended
syntax does not introduce any additional escape sequences, so [.regex]`(?[\y])` is an
unknown escape, as it would be in [.regex]`[\y]`.

In the extended syntax, [.regex]`^` does not negate a class (except within an
ordinary class nested inside an extended class); it is instead a binary
operator.

The binary operators are [.regex]`&` (intersection), [.regex]`|` or [.regex]`{plus}` (union), [.regex]`+-+`
(subtraction) and [.regex]`+^+` (symmetric difference). These are left-associative and
[.regex]`&` has higher (tighter) precedence, while the others have equal lower precedence.
The one prefix unary operator is [.regex]`!` (complement), with highest
precedence.

== UTS#18 extended character classes

The const:PCRE2_ALT_EXTENDED_CLASS option enables an alternative to Perl's [.regex]`+(?[...])+`
syntax, allowing instead extended class behaviour inside ordinary [.regex]`+[...]+`
character classes. This altered syntax for [.regex]`+[...]+` classes is loosely described
by the Unicode standard UTS#18. The const:PCRE2_ALT_EXTENDED_CLASS option does not
prevent use of [.regex]`+(?[...])+` classes; it just changes the meaning of all
[.regex]`+[...]+` classes that are not nested inside a Perl [.regex]`+(?[...])+` class.

Firstly, in ordinary Perl [.regex]`+[...]+` syntax, an expression such as [.regex]`[a[]` is a
character class with two literal characters [.regex]`a` and [.regex]`[`, but in UTS#18 extended
classes the "[" character becomes an additional metacharacter within classes,
denoting the start of a nested class, so a literal [.regex]`[` must be escaped as [.regex]`\[`.

Secondly, within the UTS#18 extended syntax, there are operators [.regex]`||`, [.regex]`&&`,
[.regex]`--` and [.regex]`~~` which denote character class union, intersection, subtraction,
and symmetric difference respectively. In standard Perl syntax, these would
simply be needlessly-repeated literals (except for [.regex]`--` which could be the
start or end of a range). In UTS#18 extended classes these operators can be used
in constructs such as [.regex]`[\p{L}--[QW]]` for "Unicode letters, other than Q and W".
A literal [.regex]`-` at the start or end of a range must be escaped, so while
[.regex]`[--1]` in Perl syntax is the range from hyphen to 1, it must be
escaped as [.regex]`[\--1]`
in UTS#18 extended classes.

Unlike Perl's [.regex]`+(?[...])+` extended classes, the const:PCRE2_EXTENDED_MORE option to
ignore space and tab characters is not automatically enabled for UTS#18
extended classes, but it is honoured if set.

Extended UTS#18 classes can be nested, and nested classes are themselves
extended classes (unlike Perl, where nested classes must be simple classes).
For example, [.regex]`[\p{L}&&[\p{Thai}||\p{Greek}]]` matches any letter that is in
the Thai or Greek scripts. Note that this means that no special grouping
characters (such as the parentheses used in Perl's [.regex]`+(?[...])+` class syntax) are
needed.

Individual class items (literal characters, literal ranges, properties such as
[.regex]`+\d+` or [.regex]`+\p{...}+`, and nested classes) can be combined by juxtaposition or by an
operator. Juxtaposition is the implicit union operator, and binds more tightly
than any explicit operator. Thus a sequence of literals and/or ranges behaves
as if it is enclosed in square brackets. For example, [.regex]`[A-Z0-9&&[^E8]]` is the
same as [.regex]`[[A-Z0-9]&&[^E8]]`, which matches any upper case alphanumeric character
except "E" or "8".

Precedence between the explicit operators is not defined, so mixing operators
is a syntax error. For example, [.regex]`+[A&&B--C]+` is an error, but
[.regex]`+[A&&[B--C]]+` is valid.

This is an emerging syntax which is being adopted gradually across the regex
ecosystem: for example JavaScript adopted the [.regex]`/v` flag in ECMAScript 2024;
Python's `re` module reserves the syntax for future use with a FutureWarning
for unescaped use of [.regex]`[` as a literal within character classes. Due to UTS#18
providing insufficient guidance, engines interpret the syntax differently.
Rust's `regex` crate and Python's `regex` PyPi module both implement UTS#18
extended classes, but with slight incompatibilities ([.regex]`[A||B&&C]` is parsed as
[.regex]`[A||[B&&C]]` in Python's `regex` but as [.regex]`[[A||B]&&C]` in Rust's `regex`).

PCRE2's syntax adds syntax restrictions similar to ECMAScript's [.regex]`/v` flag, so
that all the UTS#18 extended classes accepted as valid by PCRE2 have the
property that they are interpreted either with the same behaviour, or as
invalid, by all other major engines. Please file an issue if you are aware of
cross-engine differences in behaviour between PCRE2 and another major engine.

== POSIX character classes

Perl supports the POSIX notation for character classes. This uses names
enclosed by [.regex]`[:` and [.regex]`:]` within the enclosing square brackets. PCRE2 also supports
this notation, in both ordinary and extended classes. For example,

[source,regex]
----
[01[:alpha:]%]
----

matches [.subject]`0`, [.subject]`1`, any alphabetic character, or [.subject]`%`. The supported class names
are:

[.regex]`[:alnum:]`:: Letters and digits.
[.regex]`[:alpha:]`:: Letters.
[.regex]`[:ascii:]`:: Character codes 0--127.
[.regex]`[:blank:]`:: Space or tab only.
[.regex]`[:cntrl:]`:: Control characters.
[.regex]`[:digit:]`:: Decimal digits (same as [.regex]`\d`).
[.regex]`[:graph:]`:: Printing characters, excluding space.
[.regex]`[:lower:]`:: Lower case letters.
[.regex]`[:print:]`:: Printing characters, including space.
[.regex]`[:punct:]`:: Printing characters, excluding letters, digits, and space.
[.regex]`[:space:]`:: White space (the same as [.regex]`\s` from PCRE2 8.34).
[.regex]`[:upper:]`:: Upper case letters.
[.regex]`[:word:]`:: "Word" characters (same as [.regex]`\w`).
[.regex]`[:xdigit:]`:: Hexadecimal digits.

The default "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13),
and space (32). If locale-specific matching is taking place, the list of space
characters may be different; there may be fewer or more of them. "Space" and
[.regex]`+\s+` match the same set of characters, as do "word" and [.regex]`+\w+`.

The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
5.8. Another Perl extension is negation, which is indicated by a ^ character
after the colon. For example,

[source,regex]
----
[12[:^digit:]]
----

matches [.subject]`1`, [.subject]`2`, or any non-digit. PCRE2 (and Perl) also recognize the POSIX
syntax [.regex]`[.ch.]` and [.regex]`[=ch=]` where [.subject]`ch` is a "collating element", but these are not
supported, and an error is given if they are encountered.

By default, characters with values greater than 127 do not match any of the
POSIX character classes, although this may be different for characters in the
range 128--255 when locale-specific matching is happening. However, in UCP mode,
unless certain options are set (see below), some of the classes are changed so
that Unicode character properties are used. This is achieved by replacing
POSIX classes with other sequences, as follows:

[.regex]`[:alnum:]`:: Becomes [.regex]`\p{Xan}`
[.regex]`[:alpha:]`:: Becomes [.regex]`\p{L}`
[.regex]`[:blank:]`:: Becomes [.regex]`\h`
[.regex]`[:cntrl:]`:: Becomes [.regex]`\p{Cc}`
[.regex]`[:digit:]`:: Becomes [.regex]`\p{Nd}`
[.regex]`[:lower:]`:: Becomes [.regex]`\p{Ll}`
[.regex]`[:space:]`:: Becomes [.regex]`\p{Xps}`
[.regex]`[:upper:]`:: Becomes [.regex]`\p{Lu}`
[.regex]`[:word:]`:: Becomes [.regex]`\p{Xwd}`

Negated versions, such as [.regex]`[:^alpha:]` use [.regex]`+\P+` instead of [.regex]`+\p+`.

Four other POSIX
classes are handled specially in UCP mode:

[.regex]`[:graph:]`::
This matches characters that have glyphs that mark the page when printed. In
Unicode property terms, it matches all characters with the L, M, N, P, S, or Cf
properties, except for:

+
char:U+061C::: Arabic Letter Mark
char:U+180E::: Mongolian Vowel Separator
char:U+2066{blank}--{blank}char:U+2069::: Various "isolates"

[.regex]`[:print:]`::
This matches the same characters as [.regex]`[:graph:]` plus space characters that are
not controls, that is, characters with the Zs property.

[.regex]`[:punct:]`::
This matches all characters that have the Unicode P (punctuation) property,
plus those characters with code points less than 256 that have the S (Symbol)
property.

[.regex]`[:xdigit:]`::
In addition to the ASCII hexadecimal digits, this also matches the "fullwidth"
versions of those characters, whose Unicode code points start at char:U+FF10. This
is a change that was made in PCRE2 release 10.43 for Perl compatibility.

The other POSIX classes are unchanged by const:PCRE2_UCP, and match only characters
with code points less than 256.

There are two options that can be used to restrict the POSIX classes to ASCII
characters when const:PCRE2_UCP is set. The option const:PCRE2_EXTRA_ASCII_DIGIT affects
just [.regex]`[:digit:]` and [.regex]`[:xdigit:]`. Within a pattern, this can be set and unset by
[.regex]`+(?aT)+` and [.regex]`+(?-aT)+`. The const:PCRE2_EXTRA_ASCII_POSIX option disables UCP processing
for all POSIX classes, including [.regex]`[:digit:]` and [.regex]`[:xdigit:]`. Within a pattern,
[.regex]`+(?aP)+` and [.regex]`+(?-aP)+` set and unset both these options for consistency.

== Compatibility feature for word boundaries

In the POSIX.2 compliant library that was included in 4.4BSD Unix, the ugly
syntax [[:<:]] and [[:>:]] is used for matching "start of word" and "end of
word". PCRE2 treats these items as follows:

[.regex]`[[:<:]]`:: Is converted to [.regex]`\b(?=\w)`.
[.regex]`[[:>:]]`:: Is converted to [.regex]`+\b(?<=\w)+`.

Only these exact character sequences are recognized. A sequence such as
[a[:<:]b] provokes error for an unrecognized POSIX class name. This support is
not compatible with Perl. It is provided to help migrations from other
environments, and is best not used in any new patterns. Note that [.regex]`+\b+` matches
at the start and the end of a word (see
"Simple assertions"
above), and in a Perl-style pattern the preceding or following character
normally shows which is wanted, without the need for the assertions that are
used above in order to give exactly the POSIX behaviour. Note also that the
const:PCRE2_UCP option changes the meaning of [.regex]`\w` (and therefore [.regex]`\b`) by default, so
it also affects these POSIX sequences.

== Vertical bar (pipe)

Vertical bar characters are used to separate alternative patterns. For example,
the pattern

[source,regex]
----
gilbert|sullivan
----

matches either [.subject]`gilbert` or [.subject]`sullivan`. Any number of alternatives may appear,
and an empty alternative is permitted (matching the empty string). The matching
process tries each alternative in turn, from left to right, and the first one
that succeeds is used. If the alternatives are within a group
(defined below),
"succeeds" means matching the rest of the main pattern as well as the
alternative in the group.

[[internal-option-setting]]
== Internal option setting

The settings of several options can be changed within a pattern by a sequence
of letters enclosed between [.regex]`(?` and [.regex]`)`. The following are Perl-compatible,
and are described in detail in the
xref:pcre2api.adoc[`pcre2api`]
documentation. The option letters are:

[.regex]`i`:: For const:PCRE2_CASELESS
[.regex]`m`:: For const:PCRE2_MULTILINE
[.regex]`n`:: For const:PCRE2_NO_AUTO_CAPTURE
[.regex]`s`:: For const:PCRE2_DOTALL
[.regex]`x`:: For const:PCRE2_EXTENDED
[.regex]`xx`:: For const:PCRE2_EXTENDED_MORE

For example, [.regex]`+(?im)+` sets caseless, multiline matching. It is also possible to
unset these options by preceding the relevant letters with a hyphen, for
example [.regex]`+(?-im)+`. The two "extended" options are not independent; unsetting
either one cancels the effects of both of them.

A combined setting and unsetting such as [.regex]`+(?im-sx)+`, which sets const:PCRE2_CASELESS
and const:PCRE2_MULTILINE while unsetting const:PCRE2_DOTALL and const:PCRE2_EXTENDED, is also
permitted. Only one hyphen may appear in the options string. If a letter
appears both before and after the hyphen, the option is unset. An empty options
setting [.regex]`(?)` is allowed. Needless to say, it has no effect.

If the first character following [.regex]`(?` is a circumflex, it causes all of the above
options to be unset. Letters may follow the circumflex to cause some options to
be re-instated, but a hyphen may not appear.

Some PCRE2-specific options can be changed by the same mechanism using these
pairs or individual letters:

[.regex]`aD`:: For const:PCRE2_EXTRA_ASCII_BSD
[.regex]`aS`:: For const:PCRE2_EXTRA_ASCII_BSS
[.regex]`aW`:: For const:PCRE2_EXTRA_ASCII_BSW
[.regex]`aP`:: For const:PCRE2_EXTRA_ASCII_POSIX and const:PCRE2_EXTRA_ASCII_DIGIT
[.regex]`aT`:: For const:PCRE2_EXTRA_ASCII_DIGIT
[.regex]`r`:: For const:PCRE2_EXTRA_CASELESS_RESTRICT
[.regex]`J`:: For const:PCRE2_DUPNAMES
[.regex]`U`:: For const:PCRE2_UNGREEDY

However, except for [.regex]`r`, these are not unset by [.regex]`(?^)`, which is equivalent to
[.regex]`+(?-imnrsx)+`. If [.regex]`a` is not followed by any of the upper case letters shown
above, it sets (or unsets) all the ASCII options.

const:PCRE2_EXTRA_ASCII_DIGIT has no additional effect when const:PCRE2_EXTRA_ASCII_POSIX
is set, but including it in [.regex]`+(?aP)+` means that [.regex]`+(?-aP)+` suppresses all ASCII
restrictions for POSIX classes.

When one of these option changes occurs at top level (that is, not inside group
parentheses), the change applies until a subsequent change, or the end of the
pattern. An option change within a group (see below for a description of
groups) affects only that part of the group that follows it. At the end of the
group these options are reset to the state they were before the group. For
example,

[source,regex]
----
(a(?i)b)c
----

matches [.subject]`abc` and [.subject]`aBc` and no other strings (assuming const:PCRE2_CASELESS is not set
externally). Any changes made in one alternative do carry on into subsequent
branches within the same group. For example,

[source,regex]
----
(a(?i)b|c)
----

matches [.subject]`ab`, [.subject]`aB`, [.subject]`c`, and [.subject]`C`, even though when matching [.subject]`C` the first
branch is abandoned before the option setting. This is because the effects of
option settings happen at compile time. There would be some very weird
behaviour otherwise.

As a convenient shorthand, if any option settings are required at the start of
a non-capturing group (see the section <<groups>>), the option letters may
appear between the [.regex]`?` and the [.regex]`:`. Thus the two patterns

[source,regex]
----
(?i:saturday|sunday)
(?:(?i)saturday|sunday)
----

match exactly the same set of strings. Because alternative branches are tried
from left to right, and options are not reset until the end of the group is
reached, an option setting in one branch does affect subsequent branches, so
the above patterns match [.subject]`SUNDAY` as well as [.subject]`Saturday`.

[NOTE]
====
There are other PCRE2-specific options, applying to the whole
pattern, which can be set by the application when the compiling function is
called. In addition, the pattern can contain special leading sequences such as
[.regex]`+(*CRLF)+` to override what the application has set or what has been defaulted.
Details are given in the section <<newline-sequences>>
above. There are also the [.regex]`+(*UTF)+` and [.regex]`+(*UCP)+` leading sequences that can be used
to set UTF and Unicode property modes; they are equivalent to setting the
const:PCRE2_UTF and const:PCRE2_UCP options, respectively. However, the application can set
the const:PCRE2_NEVER_UTF or const:PCRE2_NEVER_UCP options, which lock out the use of the
[.regex]`+(*UTF)+` and [.regex]`+(*UCP)+` sequences.
====

[[groups]]
== Groups

Groups are delimited by parentheses (round brackets), which can be nested.
Turning part of a pattern into a group does two things:

. It localizes a set of alternatives. For example, the pattern

+
[source,regex]
----
cat(aract|erpillar|)
----

+
matches [.subject]`cataract`, [.subject]`caterpillar`, or [.subject]`cat`. Without the parentheses, it would
match [.subject]`cataract`, [.subject]`erpillar` or an empty string.

. It creates a "capture group". This means that, when the whole pattern
matches, the portion of the subject string that matched the group is passed
back to the caller, separately from the portion that matched the whole pattern.
(This applies only to the traditional matching function; the DFA matching
function does not support capturing.)

+
Opening parentheses are counted from left to right (starting from 1) to obtain
numbers for capture groups. For example, if the string [.subject]`the red king` is
matched against the pattern

+
[source,regex]
----
the ((red|white) (king|queen))
----

+
the captured substrings are [.subject]`red king`, [.subject]`red`, and [.subject]`king`, and are numbered 1,
2, and 3, respectively.

The fact that plain parentheses fulfil two functions is not always helpful.
There are often times when grouping is required without capturing. If an
opening parenthesis is followed by a question mark and a colon, the group
does not do any capturing, and is not counted when computing the number of any
subsequent capture groups. For example, if the string [.subject]`the white queen`
is matched against the pattern

[source,regex]
----
the ((?:red|white) (king|queen))
----

the captured substrings are [.subject]`white queen` and [.subject]`queen`, and are numbered 1 and 2. The maximum number of capture groups is macroval:MAX_GROUP_NUMBER.

As a convenient shorthand, if any option settings are required at the start of
a non-capturing group, the option letters may appear between the [.regex]`?` and the
[.regex]`:` (see the section <<groups>>).

[[duplicate-group-numbers]]
== Duplicate group numbers

Perl 5.10 introduced a feature whereby each alternative in a group uses the
same numbers for its capturing parentheses. Such a group starts with [.regex]`+(?|+` and is
itself a non-capturing group. For example, consider this pattern:

[source,regex]
----
(?|(Sat)ur|(Sun))day
----

Because the two alternatives are inside a [.regex]`+(?|+` group, both sets of capturing
parentheses are numbered one. Thus, when the pattern matches, you can look
at captured substring number one, whichever alternative matched. This construct
is useful when you want to capture part, but not all, of one of a number of
alternatives. Inside a [.regex]`+(?|+` group, parentheses are numbered as usual, but the
number is reset at the start of each branch. The numbers of any capturing
parentheses that follow the whole group start after the highest number used in
any branch. The following example is taken from the Perl documentation. The
numbers underneath show in which buffer the captured content will be stored.

[source,text]
----
# before  ---------------branch-reset----------- after
/ ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
# 1            2         2  3        2     3     4
----

A backreference to a capture group uses the most recent value that is set for
the group. The following pattern matches [.subject]`abcabc` or [.subject]`defdef`:

[source,regex]
----
(?|(abc)|(def))\1
----

In contrast, a subroutine call to a capture group always refers to the
first one in the pattern with the given number. The following pattern matches
[.subject]`abcabc` or [.subject]`defabc`:

[source,regex]
----
(?|(abc)|(def))(?1)
----

A relative reference such as [.regex]`+(?-1)+` is no different: it is just a convenient way
of computing an absolute group number.

If a
condition test
for a group's having matched refers to a non-unique number, the test is
true if any group with that number has matched.

An alternative approach to using this "branch reset" feature is to use
duplicate named groups, as described in the section <<named-capture-groups>>.

[[named-capture-groups]]
== Named capture groups

Identifying capture groups by number is simple, but it can be very hard to keep
track of the numbers in complicated patterns. Furthermore, if an expression is
modified, the numbers may change. To help with this difficulty, PCRE2 supports
the naming of capture groups. This feature was not added to Perl until release
5.10. Python had the feature earlier, and PCRE1 introduced it at release 4.0,
using the Python syntax. PCRE2 supports both the Perl and the Python syntax.

In PCRE2, a capture group can be named in one of three ways: [.regex]`(?<__name__>...)` or
[.regex]`(?'__name__'...)` as in Perl, or [.regex]`(?P<__name__>...)` as in Python. Names may be up to
macroval:MAX_NAME_SIZE code units long. When const:PCRE2_UTF is not set, they may contain only ASCII
alphanumeric characters and underscores, but must start with a non-digit. When
const:PCRE2_UTF is set, the syntax of group names is extended to allow any Unicode
letter or Unicode decimal digit. In other words, group names must match one of
these patterns:

[.regex]`^[_A-Za-z][_A-Za-z0-9]*\z`  :: when const:PCRE2_UTF is not set
[.regex]`^[_\p{L}][_\p{L}\p{Nd}]*\z` :: when const:PCRE2_UTF is set

References to capture groups from other parts of the pattern, such as
backreferences,
recursion,
and
conditions,
can all be made by name as well as by number.

Named capture groups are allocated numbers as well as names, exactly as
if the names were not present. In both PCRE2 and Perl, capture groups
are primarily identified by numbers; any names are just aliases for these
numbers. The PCRE2 API provides function calls for extracting the complete
name-to-number translation table from a compiled pattern, as well as
convenience functions for extracting captured substrings by name.

Warning: When more than one capture group has the same number, as
described in the section <<duplicate-group-numbers>>, a name given to one of them applies to all
of them. Perl allows identically numbered groups to have different names.
Consider this pattern, where there are two capture groups, both numbered 1:

[source,regex]
----
(?|(?<AA>aa)|(?<BB>bb))
----

Perl allows this, with both names AA and BB as aliases of group 1. Thus, after
a successful match, both names yield the same value (either [.subject]`aa` or [.subject]`bb`).

In an attempt to reduce confusion, PCRE2 does not allow the same group number
to be associated with more than one name. The example above provokes a
compile-time error. However, there is still scope for confusion. Consider this
pattern:

[source,regex]
----
(?|(?<AA>aa)|(bb))
----

Although the second group number 1 is not explicitly named, the name AA is
still an alias for any group 1. Whether the pattern matches [.subject]`aa` or [.subject]`bb`, a
reference by name to group AA yields the matched string.

By default, a name must be unique within a pattern, except that duplicate names
are permitted for groups with the same number, for example:

[source,regex]
----
(?|(?<AA>aa)|(?<AA>bb))
----

The duplicate name constraint can be disabled by setting the const:PCRE2_DUPNAMES
option at compile time, or by the use of [.regex]`+(?J)+` within the pattern, as described
in the section <<internal-option-setting>>
above.

Duplicate names can be useful for patterns where only one instance of the named
capture group can match. Suppose you want to match the name of a weekday,
either as a 3-letter abbreviation or as the full name, and in both cases you
want to extract the abbreviation. This pattern (ignoring the line breaks) does
the job:

[source,regex]
----
(?J)
(?<DN>Mon|Fri|Sun)(?:day)?|
(?<DN>Tue)(?:sday)?|
(?<DN>Wed)(?:nesday)?|
(?<DN>Thu)(?:rsday)?|
(?<DN>Sat)(?:urday)?
----

There are five capture groups, but only one is ever set after a match. The
convenience functions for extracting the data by name returns the substring for
the first (and in this example, the only) group of that name that matched. This
saves searching to find which numbered group it was. (An alternative way of
solving this problem is to use a "branch reset" group, as described in the
section <<duplicate-group-numbers>>.)

If you make a backreference to a non-unique named group from elsewhere in the
pattern, the groups to which the name refers are checked in the order in which
they appear in the overall pattern. The first one that is set is used for the
reference. For example, this pattern matches both [.subject]`foofoo` and [.subject]`barbar` but not
[.subject]`foobar` or [.subject]`barfoo`:

[source,regex]
----
(?J)(?:(?<n>foo)|(?<n>bar))\k<n>
----

If you make a subroutine call to a non-unique named group, the one that
corresponds to the first occurrence of the name is used. In the absence of
duplicate numbers this is the one with the lowest number.

If you use a named reference in a condition
test (see the
section <<conditional-groups>>
below), either to check whether a capture group has matched, or to check for
recursion, all groups with the same name are tested. If the condition is true
for any one of them, the overall condition is true. This is the same behaviour
as testing by number. For further details of the interfaces for handling named
capture groups, see the
xref:pcre2api.adoc[`pcre2api`]
documentation.

== Repetition

Repetition is specified by quantifiers, which may follow any one of these
items:

* a literal data character
* the dot metacharacter
* the [.regex]`\C` escape sequence
* the [.regex]`\R` escape sequence
* the [.regex]`\X` escape sequence
* any escape sequence that matches a single character
* a character class
* a backreference
* a parenthesized group (including lookaround assertions)
* a subroutine call (recursive or otherwise)

If a quantifier does not follow a repeatable item, an error occurs. The
general repetition quantifier specifies a minimum and maximum number of
permitted matches by giving two numbers in curly brackets (braces), separated
by a comma. The numbers must be no greater than macroval:MAX_REPEAT_COUNT, and the first must be less
than or equal to the second. For example,

[source,regex]
----
z{2,4}
----

matches [.subject]`zz`, [.subject]`zzz`, or [.subject]`zzzz`. A closing brace on its own is not a special
character. If the second number is omitted, but the comma is present, there is
no upper limit; if the second number and the comma are both omitted, the
quantifier specifies an exact number of required matches. Thus

[source,regex]
----
[aeiou]{3,}
----

matches at least 3 successive vowels, but may match many more, whereas

[source,regex]
----
\d{8}
----

matches exactly 8 digits. If the first number is omitted, the lower limit is
taken as zero; in this case the upper limit must be present.

[.regex]`X{,4}` is interpreted as [.regex]`X{0,4}`.

This is a change in behaviour that happened in Perl 5.34.0 and PCRE2 10.43. In
earlier versions such a sequence was not interpreted as a quantifier. Other
regular expression engines may behave either way.

If the characters that follow an opening brace do not match the syntax of a
quantifier, the brace is taken as a literal character. In particular, this
means that [.regex]`{,}` is a literal string of three characters.

Note that not every opening brace is potentially the start of a quantifier
because braces are used in other items such as [.regex]`\N{U+345}` or [.regex]`\k{_name_}`.

In UTF modes, quantifiers apply to characters rather than to individual code
units. Thus, for example, [.regex]`+\x{100}{2}+` matches two characters, each of
which is represented by a two-byte sequence in a UTF-8 string. Similarly,
[.regex]`+\X{3}+` matches three Unicode extended grapheme clusters, each of which may be
several code units long (and they may be of different lengths).

The quantifier [.regex]`{0}` is permitted, causing the expression to behave as if the
previous item and the quantifier were not present. This may be useful for
capture groups that are referenced as
subroutines
from elsewhere in the pattern (but see also the section
<<defining-capture-groups>>
below). Except for parenthesized groups, items that have a [.regex]`{0}` quantifier are
omitted from the compiled pattern.

For convenience, the three most common quantifiers have single-character
abbreviations:

[.regex]`+*+`:: Is equivalent to [.regex]`{0,}`.
[.regex]`+`:: Is equivalent to [.regex]`{1,}`.
[.regex]`+?+`:: Is equivalent to [.regex]`{0,1}`.

It is possible to construct infinite loops by following a group that can match
no characters with a quantifier that has no upper limit, for example:

[source,regex]
----
(a?)*
----

Earlier versions of Perl and PCRE1 used to give an error at compile time for
such patterns. However, because there are cases where this can be useful, such
patterns are now accepted, but whenever an iteration of such a group matches no
characters, matching moves on to the next item in the pattern instead of
repeatedly matching an empty string. This does not prevent backtracking into
any of the iterations if a subsequent item fails to match.

By default, quantifiers are "greedy", that is, they match as much as possible
(up to the maximum number of permitted repetitions), without causing the rest
of the pattern to fail. The classic example of where this gives problems is in
trying to match comments in C programs. These appear between [.c]`+/*+` and [.c]`+*/+` and
within the comment, individual `+*+` and `/` characters may appear. An attempt to
match C comments by applying the pattern

[source,regex]
----
/\*.*\*/
----

to the string

[source,text]
----
/* first comment */  not comment  /* second comment */
----

fails, because it matches the entire string owing to the greediness of the [.regex]`+.*+`
item. However, if a quantifier is followed by a question mark, it ceases to be
greedy, and instead matches the minimum number of times possible, so the
pattern

[source,regex]
----
/\*.*?\*/
----

does the right thing with C comments. The meaning of the various quantifiers is
not otherwise changed, just the preferred number of matches. Do not confuse
this use of question mark with its use as a quantifier in its own right.
Because it has two uses, it can sometimes appear doubled, as in

[source,regex]
----
\d??\d
----

which matches one digit by preference, but can match two if that is the only
way the rest of the pattern matches.

If the const:PCRE2_UNGREEDY option is set (an option that is not available in Perl),
the quantifiers are not greedy by default, but individual ones can be made
greedy by following them with a question mark. In other words, it inverts the
default behaviour.

When a parenthesized group is quantified with a minimum repeat count that
is greater than 1 or with a limited maximum, more memory is required for the
compiled pattern, in proportion to the size of the minimum or maximum.

If a pattern starts with [.regex]`+.*+` or [.regex]`+.{0,}+` and the const:PCRE2_DOTALL option (equivalent
to Perl's [.regex]`/s`) is set, thus allowing the dot to match newlines, the pattern is
implicitly anchored, because whatever follows will be tried against every
character position in the subject string, so there is no point in retrying the
overall match at any position after the first. PCRE2 normally treats such a
pattern as though it were preceded by [.regex]`+\A+`.

In cases where it is known that the subject string contains no newlines, it is
worth setting const:PCRE2_DOTALL in order to obtain this optimization, or
alternatively, using [.regex]`^` to indicate anchoring explicitly.

However, there are some cases where the optimization cannot be used. When [.regex]`+.*+`
is inside capturing parentheses that are the subject of a backreference
elsewhere in the pattern, a match at the start may fail where a later one
succeeds. Consider, for example:

[source,regex]
----
(.*)abc\1
----

If the subject is [.subject]`xyz123abc123` the match point is the fourth character. For
this reason, such a pattern is not implicitly anchored.

Another case where implicit anchoring is not applied is when the leading [.regex]`+.*+` is
inside an atomic group. Once again, a match at the start may fail where a later
one succeeds. Consider this pattern:

[source,regex]
----
(?>.*?a)b
----

It matches [.subject]`ab` in the subject [.subject]`aab`. The use of the backtracking control verbs
[.regex]`+(*PRUNE)+` and [.regex]`+(*SKIP)+` also disable this optimization. To do so explicitly,
either pass the compile option const:PCRE2_NO_DOTSTAR_ANCHOR, or call
func:pcre2_set_optimize() with a const:PCRE2_DOTSTAR_ANCHOR_OFF directive.

When a capture group is repeated, the value captured is the substring that
matched the final iteration. For example, after

[source,regex]
----
(tweedle[dume]{3}\s*)+
----

has matched [.subject]`tweedledum tweedledee` the value of the captured substring is
[.subject]`tweedledee`. However, if there are nested capture groups, the corresponding
captured values may have been set in previous iterations. For example, after

[source,regex]
----
(a|(b))+
----

matches [.subject]`aba` the value of the second captured substring is [.subject]`b`.

== Atomic grouping and possessive quantifiers

With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
repetition, failure of what follows normally causes the repeated item to be
re-evaluated to see if a different number of repeats allows the rest of the
pattern to match. Sometimes it is useful to prevent this, either to change the
nature of the match, or to cause it fail earlier than it otherwise might, when
the author of the pattern knows there is no point in carrying on.

Consider, for example, the pattern [.regex]`\d+foo` when applied to the subject line
[.subject]`123456bar`.

After matching all 6 digits and then failing to match [.subject]`foo`, the normal
action of the matcher is to try again with only 5 digits matching the [.regex]`\d{plus}`
item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
(a term taken from Jeffrey Friedl's book) provides the means for specifying
that once a group has matched, it is not to be re-evaluated in this way.

If we use atomic grouping for the previous example, the matcher gives up
immediately on failing to match [.subject]`foo` the first time. The notation is a kind of
special parenthesis, starting with [.regex]`(?>` as in this example:

[source,regex]
----
(?>\d+)foo
----

Perl 5.28 introduced an experimental alphabetic form starting with [.regex]`+(*+` which may
be easier to remember:

[source,regex]
----
(*atomic:\d+)foo
----

This kind of parenthesized group "locks up" the part of the pattern it contains
once it has matched, and a failure further into the pattern is prevented from
backtracking into it. Backtracking past it to previous items, however, works as
normal.

An alternative description is that a group of this type matches exactly the
string of characters that an identical standalone pattern would match, if
anchored at the current point in the subject string.

Atomic groups are not capture groups. Simple cases such as the above example
can be thought of as a maximizing repeat that must swallow everything it can.
So, while both [.regex]`\d{plus}` and [.regex]`\d+?` are prepared to adjust the number of digits they
match in order to make the rest of the pattern match, [.regex]`(?>\d+)` can only match
an entire sequence of digits.

Atomic groups in general can of course contain arbitrarily complicated
expressions, and can be nested. However, when the contents of an atomic
group is just a single repeated item, as in the example above, a simpler
notation, called a "possessive quantifier" can be used. This consists of an
additional [.regex]`+` character following a quantifier. Using this notation, the
previous example can be rewritten as

[source,regex]
----
\d++foo
----

Note that a possessive quantifier can be used with an entire group, for
example:

[source,regex]
----
(abc|xyz){2,3}+
----

Possessive quantifiers are always greedy; the setting of the const:PCRE2_UNGREEDY
option is ignored. They are a convenient notation for the simpler forms of
atomic group. However, there is no difference in the meaning of a possessive
quantifier and the equivalent atomic group, though there may be a performance
difference; possessive quantifiers should be slightly faster.

The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
Jeffrey Friedl originated the idea (and the name) in the first edition of his
book. Mike McCloskey liked it, so implemented it when he built Sun's Java
package, and PCRE1 copied it from there. It found its way into Perl at release
5.10.

PCRE2 has an optimization that automatically "possessifies" certain simple
pattern constructs. For example, the sequence [.regex]`A{plus}B` is treated as [.regex]`A{plus}{plus}B` because
there is no point in backtracking into a sequence of A's when B must follow.
This feature can be disabled by the const:PCRE2_NO_AUTO_POSSESS option, by calling
func:pcre2_set_optimize() with a const:PCRE2_AUTO_POSSESS_OFF directive, or by
starting the pattern with [.regex]`+(*NO_AUTO_POSSESS)+`.

When a pattern contains an unlimited repeat inside a group that can itself be
repeated an unlimited number of times, the use of an atomic group is the only
way to avoid some failing matches taking a very long time indeed. The pattern

[source,regex]
----
(\D+|<\d+>)*[!?]
----

matches an unlimited number of substrings that either consist of non-digits, or
digits enclosed in <>, followed by either ! or ?. When it matches, it runs
quickly. However, if it is applied to

[source,text]
----
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
----

it takes a long time before reporting failure. This is because the string can
be divided between the internal [.regex]`\D{plus}` repeat and the external [.regex]`+*+` repeat in a
large number of ways, and all have to be tried. (The example uses [.regex]`[!?]` rather
than a single character at the end, because both PCRE2 and Perl have an
optimization that allows for fast failure when a single character is used. They
remember the last single character that is required for a match, and fail early
if it is not present in the string.) If the pattern is changed so that it uses
an atomic group, like this:

[source,regex]
----
((?>\D+)|<\d+>)*[!?]
----

sequences of non-digits cannot be broken, and failure happens quickly.

== Backreferences

Outside a character class, a backslash followed by a digit greater than 0 (and
possibly further digits) is a backreference to a capture group earlier (that
is, to its left) in the pattern, provided there have been that many previous
capture groups.

However, if the decimal number following the backslash is less than 8, it is
always taken as a backreference, and causes an error only if there are not that
many capture groups in the entire pattern. In other words, the group that is
referenced need not be to the left of the reference for numbers less than 8. A
"forward backreference" of this type can make sense when a repetition is
involved and the group to the right has participated in an earlier iteration.

It is not possible to have a numerical "forward backreference" to a group whose
number is 8 or more using this syntax because a sequence such as [.regex]`\50` is
interpreted as a character defined in octal. See the subsection
<<non-printing-characters>>
above
for further details of the handling of digits following a backslash. Other
forms of backreferencing do not suffer from this restriction. In particular,
there is no problem when named capture groups are used (see below).

Another way of avoiding the ambiguity inherent in the use of digits following a
backslash is to use the [.regex]`+\g+` escape sequence. This escape must be followed by a
signed or unsigned number, optionally enclosed in braces. These examples are
all identical ways of matching [.subject]`ring, ring`:

[source,regex]
----
(ring), \1
(ring), \g1
(ring), \g{1}
----

An unsigned number specifies an absolute reference without the ambiguity that
is present in the older syntax. It is also useful when literal digits follow
the reference. A signed number is a relative reference. Consider this example:

[source,regex]
----
(abc(def)ghi)\g{-1}
----

The sequence [.regex]`+\g{-1}+` is a reference to the capture group whose number is one
less than the number of the next group to be started, so in this example (where
the next group would be numbered 3) is it equivalent to [.regex]`\2`, and [.regex]`+\g{-2}+` would
be equivalent to [.regex]`\1`. Note that if this construct is inside a capture group,
that group is included in the count, so in this example [.regex]`+\g{-2}+` also refers to
group 1:

[source,regex]
----
(A)(\g{-2}B)
----

The use of relative references can be helpful in long patterns, and also in
patterns that are created by joining together fragments that contain references
within themselves.

The sequence [.regex]`\g{+1}` is a reference to the next capture group that is started
after this item, and [.regex]`\g{+2}` refers to the one after that, and so on. This kind
of forward reference can be useful in patterns that repeat. Perl does not
support the use of [.regex]`{plus}` in this way.

A backreference matches whatever actually most recently matched the capture
group in the current subject string, rather than anything at all that matches
the group (see
"Groups as subroutines"
below for a way of doing that). So the pattern

[source,regex]
----
(sens|respons)e and \1ibility
----

matches [.subject]`sense and sensibility` and [.subject]`response and responsibility`, but not
"sense and responsibility". If caseful matching is in force at the time of the
backreference, the case of letters is relevant. For example,

[source,regex]
----
((?i)rah)\s+\1
----

matches [.subject]`rah rah` and [.subject]`RAH RAH`, but not [.subject]`RAH rah`, even though the original
capture group is matched caselessly.

There are several different ways of writing backreferences to named capture
groups. The .NET syntax is [.regex]`\k{_name_}`, the Python syntax is [.regex]`(?=__name__)`, and the
original Perl syntax is [.regex]`\k<__name__>` or [.regex]`\k'__name__'`. All of these are now supported
by both Perl and PCRE2. Perl 5.10's unified backreference syntax, in which [.regex]`+\g+`
can be used for both numeric and named references, is also supported by PCRE2.
We could rewrite the above example in any of the following ways:

[source,regex]
----
(?<p1>(?i)rah)\s+\k<p1>
(?'p1'(?i)rah)\s+\k{p1}
(?P<p1>(?i)rah)\s+(?P=p1)
(?<p1>(?i)rah)\s+\g{p1}
----

A capture group that is referenced by name may appear in the pattern before or
after the reference.

There may be more than one backreference to the same group. If a group has not
actually been used in a particular match, backreferences to it always fail by
default. For example, the pattern

[source,regex]
----
(a|(bc))\2
----

always fails if it starts to match [.subject]`a` rather than [.subject]`bc`. However, if the
const:PCRE2_MATCH_UNSET_BACKREF option is set at compile time, a backreference to an
unset value matches an empty string.

Because there may be many capture groups in a pattern, all digits following a
backslash are taken as part of a potential backreference number. If the pattern
continues with a digit character, some delimiter must be used to terminate the
backreference. If the const:PCRE2_EXTENDED or const:PCRE2_EXTENDED_MORE option is set, this
can be white space. Otherwise, the [.regex]`+\g{}+` syntax or an empty comment (see
"Comments"
below) can be used.

=== Recursive backreferences

A backreference that occurs inside the group to which it refers fails when the
group is first used, so, for example, (a\1) never matches. However, such
references can be useful inside repeated groups. For example, the pattern

[source,regex]
----
(a|b\1)+
----

matches any number of [.subject]`a` characters and also [.subject]`aba`, [.subject]`ababbaa`, etc. At each iteration of
the group, the backreference matches the character string corresponding to the
previous iteration. In order for this to work, the pattern must be such that
the first iteration does not need to match the backreference. This can be done
using alternation, as in the example above, or by a quantifier with a minimum
of zero.

For versions of PCRE2 less than 10.25, backreferences of this type used to
cause the group that they reference to be treated as an
atomic group.
This restriction no longer applies, and backtracking into such groups can occur
as normal.

== Assertions

An assertion is a test that does not consume any characters. The test must
succeed for the match to continue. The simple assertions coded as [.regex]`+\b+`, [.regex]`+\B+`,
[.regex]`+\A+`, [.regex]`+\G+`, [.regex]`+\Z+`, [.regex]`+\z+`, [.regex]`^` and [.regex]`$` are described
above.

More complicated assertions are coded as parenthesized groups. If matching such
a group succeeds, matching continues after it, but with the matching position
in the subject string reset to what it was before the assertion was processed.

A special kind of assertion, called a "scan substring" assertion, matches a
subpattern against a previously captured substring. This is described in the
section <<scan-substring-assertions>>
below. It is a PCRE2 extension, not compatible with Perl.

The other goup-based assertions are of two kinds: those that look ahead of the
current position in the subject string, and those that look behind it, and in
each case an assertion may be positive (must match for the assertion to be
true) or negative (must not match for the assertion to be true).

The Perl-compatible lookaround assertions are atomic. If an assertion is true,
but there is a subsequent matching failure, there is no backtracking into the
assertion. However, there are some cases where non-atomic assertions can be
useful. PCRE2 has some support for these, described in the section
<<non-atomic-assertions>>
below, but they are not Perl-compatible.

A lookaround assertion may appear as the condition in a
conditional group
(see below). In this case, the result of matching the assertion determines
which branch of the condition is followed.

Assertion groups are not capture groups. If an assertion contains capture
groups within it, these are counted for the purposes of numbering the capture
groups in the whole pattern. Within each branch of an assertion, locally
captured substrings may be referenced in the usual way. For example, a sequence
such as [.regex]`+(.)\g{-1}+` can be used to check that two adjacent characters are the
same.

When a branch within an assertion fails to match, any substrings that were
captured are discarded (as happens with any pattern branch that fails to
match). A negative assertion is true only when all its branches fail to match;
this means that no captured substrings are ever retained after a successful
negative assertion. When an assertion contains a matching branch, what happens
depends on the type of assertion.

For a positive assertion, internally captured substrings in the successful
branch are retained, and matching continues with the next pattern item after
the assertion. For a negative assertion, a matching branch means that the
assertion is not true. If such an assertion is being used as a condition in a
conditional group
(see below), captured substrings are retained, because matching continues with
the "no" branch of the condition. For other failing negative assertions,
control passes to the previous backtracking point, thus discarding any captured
strings within the assertion.

Most assertion groups may be repeated; though it makes no sense to assert the
same thing several times, the side effect of capturing in positive assertions
may occasionally be useful. However, an assertion that forms the condition for
a conditional group may not be quantified. PCRE2 used to restrict the
repetition of assertions, but from release 10.35 the only restriction is that
an unlimited maximum repetition is changed to be one more than the minimum. For
example, [.regex]`{3,}` is treated as [.regex]`{3,4}`.

=== Alphabetic assertion names

Traditionally, symbolic sequences such as [.regex]`+(?=+` and [.regex]`+(?<=+` have been used to
specify lookaround assertions. Perl 5.28 introduced some experimental
alphabetic alternatives which might be easier to remember. They all start with
[.regex]`+(*+` instead of [.regex]`+(?+` and must be written using lower case letters. PCRE2 supports
the following synonyms:

[.regex]`+(*positive_lookahead:+`::
[.regex]`+(*pla:+`::
    Is the same as [.regex]`+(?=+`.
[.regex]`+(*negative_lookahead:+`::
[.regex]`+(*nla:+`::
    Is the same as [.regex]`+(?!+`.
[.regex]`+(*positive_lookbehind:+`::
[.regex]`+(*plb:+`::
    Is the same as [.regex]`+(?<=+`.
[.regex]`+(*negative_lookbehind:+`::
[.regex]`+(*nlb:+`::
    Is the same as [.regex]`+(?<!+`.

For example, [.regex]`(*pla:__foo__)` is the same assertion as [.regex]`(?=__foo__)`. In the following
sections, the various assertions are described using the original symbolic
forms.

=== Lookahead assertions

Lookahead assertions start with [.regex]`+(?=+` for positive assertions and [.regex]`+(?!+` for
negative assertions. For example,

[source,regex]
----
\w+(?=;)
----

matches a word followed by a semicolon, but does not include the semicolon in
the match, and

[source,regex]
----
foo(?!bar)
----

matches any occurrence of [.subject]`foo` that is not followed by [.subject]`bar`. Note that the
apparently similar pattern

[source,regex]
----
(?!foo)bar
----

does not find an occurrence of [.subject]`bar` that is preceded by something other than
[.subject]`foo`; it finds any occurrence of [.subject]`bar` whatsoever, because the assertion
[.regex]`+(?!foo)+` is always true when the next three characters are [.subject]`bar`. A
lookbehind assertion is needed to achieve the other effect.

If you want to force a matching failure at some point in a pattern, the most
convenient way to do it is with [.regex]`+(?!)+` because an empty string always matches, so
an assertion that requires there not to be an empty string must always fail.
The backtracking control verb [.regex]`+(*FAIL)+` or [.regex]`+(*F)+` is a synonym for [.regex]`+(?!)+`.

=== Lookbehind assertions

Lookbehind assertions start with [.regex]`+(?<=+` for positive assertions and [.regex]`+(?<!+` for
negative assertions. For example,

[source,regex]
----
(?<!foo)bar
----

does find an occurrence of [.subject]`bar` that is not preceded by [.subject]`foo`. The contents of
a lookbehind assertion are restricted such that there must be a known maximum
to the lengths of all the strings it matches. There are two cases:

If every top-level alternative matches a fixed length, for example

[source,regex]
----
(?<=colour|color)
----

there is a limit of macroval:LOOKBEHIND_MAX characters to the lengths, which do not have to be
the same, as this example demonstrates. This is the only kind of lookbehind
supported by PCRE2 versions earlier than 10.43 and by the alternative matching
function func:pcre2_dfa_match().

In PCRE2 10.43 and later, func:pcre2_match() supports lookbehind assertions in
which one or more top-level alternatives can match more than one string length,
for example

[source,regex]
----
(?<=colou?r)
----

The maximum matching length for any branch of the lookbehind is limited to a
value set by the calling program (default macroval:MAX_VARLOOKBEHIND characters). Unlimited repetition
(for example [.regex]`+\d*+`) is not supported. In some cases, the escape sequence [.regex]`+\K+`
(see above)
can be used instead of a lookbehind assertion at the start of a pattern to get
round the length limit restriction.

In UTF-8 and UTF-16 modes, PCRE2 does not allow the [.regex]`+\C+` escape (which matches a
single code unit even in a UTF mode) to appear in lookbehind assertions,
because it makes it impossible to calculate the length of the lookbehind. The
[.regex]`+\X+` and [.regex]`+\R+` escapes, which can match different numbers of code units, are never
permitted in lookbehinds.

"Subroutine"
calls (see below) such as [.regex]`(?2)` or [.regex]`(?&X)` are permitted in lookbehinds, as long
as the called capture group matches a limited-length string. However,
recursion,
that is, a "subroutine" call into a group that is already active,
is not supported.

PCRE2 supports backreferences in lookbehinds, but only if certain conditions
are met. The const:PCRE2_MATCH_UNSET_BACKREF option must not be set, there must be no
use of [.regex]`+(?|+` in the pattern (it creates duplicate group numbers), and if the
backreference is by name, the name must be unique. Of course, the referenced
group must itself match a limited length substring. The following pattern
matches words containing at least two characters that begin and end with the
same character:

[source,regex]
----
 \b(\w)\w++(?<=\1)
----

Possessive quantifiers can be used in conjunction with lookbehind assertions to
specify efficient matching at the end of subject strings. Consider a simple
pattern such as

[source,regex]
----
abcd$
----

when applied to a long string that does not match. Because matching proceeds
from left to right, PCRE2 will look for each [.subject]`a` in the subject and then see if
what follows matches the rest of the pattern. If the pattern is specified as

[source,regex]
----
^.*abcd$
----

the initial [.regex]`+.*+` matches the entire string at first, but when this fails (because
there is no following [.subject]`a`), it backtracks to match all but the last character,
then all but the last two characters, and so on. Once again the search for [.subject]`a`
covers the entire string, from right to left, so we are no better off. However,
if the pattern is written as

[source,regex]
----
^.*+(?<=abcd)
----

there can be no backtracking for the [.regex]`.*{plus}` item because of the possessive
quantifier; it can match only the entire string. The subsequent lookbehind
assertion does a single test on the last four characters. If it fails, the
match fails immediately. For long strings, this approach makes a significant
difference to the processing time.

=== Using multiple assertions

Several assertions (of any sort) may occur in succession. For example,

[source,regex]
----
(?<=\d{3})(?<!999)foo
----

matches [.subject]`foo` preceded by three digits that are not [.subject]`999`. Notice that each of
the assertions is applied independently at the same point in the subject
string. First there is a check that the previous three characters are all
digits, and then there is a check that the same three characters are not [.subject]`999`.
This pattern does _not_ match [.subject]`foo` preceded by six characters, the first
of which are digits and the last three of which are not [.subject]`999`. For example, it
doesn't match [.subject]`123abcfoo`. A pattern to do that is

[source,regex]
----
(?<=\d{3}...)(?<!999)foo
----

This time the first assertion looks at the preceding six characters, checking
that the first three are digits, and then the second assertion checks that the
preceding three characters are not [.subject]`999`.

Assertions can be nested in any combination. For example,

[source,regex]
----
(?<=(?<!foo)bar)baz
----

matches an occurrence of [.subject]`baz` that is preceded by [.subject]`bar` which in turn is not
preceded by [.subject]`foo`, while

[source,regex]
----
(?<=\d{3}(?!999)...)foo
----

is another pattern that matches [.subject]`foo` preceded by three digits and any three
characters that are not [.subject]`999`.

[[non-atomic-assertions]]
== Non-atomic assertions

Traditional lookaround assertions are atomic. That is, if an assertion is true,
but there is a subsequent matching failure, there is no backtracking into the
assertion. However, there are some cases where non-atomic positive assertions
can be useful. PCRE2 provides these using the following syntax:

[.regex]`+(*non_atomic_positive_lookahead:+`::
[.regex]`+(*napla:+`::
[.regex]`+(?*+`::
    Non-atomic positive lookahead.
[.regex]`+(*non_atomic_positive_lookbehind:+`::
[.regex]`+(*naplb:+`::
[.regex]`+(?<*+`::
    Non-atomic positive lookbehind.

Consider the problem of finding the right-most word in a string that also
appears earlier in the string, that is, it must appear at least twice in total.
This pattern returns the required result as captured substring 1:

[source,regex]
----
^(?x)(*napla: .* \b(\w++)) (?> .*? \b\1\b ){2}
----

For a subject such as [.subject]`word1 word2 word3 word2 word3 word4` the result is
[.subject]`word3`. How does it work? At the start, [.regex]`^(?x)` anchors the pattern and sets the
[.regex]`x` option, which causes white space (introduced for readability) to be
ignored. Inside the assertion, the greedy [.regex]`+.*+` at first consumes the entire
string, but then has to backtrack until the rest of the assertion can match a
word, which is captured by group 1. In other words, when the assertion first
succeeds, it captures the right-most word in the string.

The current matching point is then reset to the start of the subject, and the
rest of the pattern match checks for two occurrences of the captured word,
using an ungreedy [.regex]`+.*?+` to scan from the left. If this succeeds, we are done, but
if the last word in the string does not occur twice, this part of the pattern
fails. If a traditional atomic lookahead [.regex]`+(?=+` or [.regex]`+(*pla:+` had been used, the
assertion could not be re-entered, and the whole match would fail. The pattern
would succeed only if the very last word in the subject was found twice.

Using a non-atomic lookahead, however, means that when the last word does not
occur twice in the string, the lookahead can backtrack and find the second-last
word, and so on, until either the match succeeds, or all words have been
tested.

Two conditions must be met for a non-atomic assertion to be useful: the
contents of one or more capturing groups must change after a backtrack into the
assertion, and there must be a backreference to a changed group later in the
pattern. If this is not the case, the rest of the pattern match fails exactly
as before because nothing has changed, so using a non-atomic assertion just
wastes resources.

There is one exception to backtracking into a non-atomic assertion. If an
[.regex]`+(*ACCEPT)+` control verb is triggered, the assertion succeeds atomically. That
is, a subsequent match failure cannot backtrack into the assertion.

Non-atomic assertions are not supported by the alternative matching function
func:pcre2_dfa_match(). They are supported by JIT, but only if they do not
contain any control verbs such as [.regex]`+(*ACCEPT)+`. (This may change in future). Note
that assertions that appear as conditions for
conditional groups
(see below) must be atomic.

[[scan-substring-assertions]]
== Scan substring assertions

A special kind of assertion, not compatible with Perl, makes it possible to
check the contents of a captured substring by matching it with a subpattern.
Because this involves capturing, this feature is not supported by
func:pcre2_dfa_match().

A scan substring assertion starts with the sequence [.regex]`+(*scan_substring:+` or
[.regex]`+(*scs:+` which is followed by a list of substring numbers (absolute or relative)
and/or substring names enclosed in single quotes or angle brackets, all within
parentheses. The rest of the item is the subpattern that is applied to the
substring, as shown in these examples:

[source,regex]
----
(*scan_substring:(1)...)
(*scs:(-2)...)
(*scs:('AB')...)
(*scs:(1,'AB',-2)...)
----

The list of groups is checked in the order they are given, and it is the
contents of the first one that is found to be set that are scanned. When
const:PCRE2_DUPNAMES is set and there are ambiguous group names, all groups with the
same name are checked in numerical order. A scan substring assertion fails if
none of the groups it references have been set.

The pattern match on the substring is always anchored, that is, it must match
from the start of the substring. There is no "bumpalong" if it does not match
at the start. The end of the subject is temporarily reset to be the end of the
substring, so [.regex]`+\Z+`, [.regex]`+\z+`, and [.regex]`+$+` will match there. However, the start of the
subject is _not_ reset. This means that [.regex]`+^+` matches only if the substring is
actually at the start of the main subject, but it also means that lookbehind
assertions into what precedes the substring are possible.

Here is a very simple example: find a word that contains the rare (in English)
sequence of letters [.subject]`rh` not at the start:

[source,regex]
----
\b(\w++)(*scs:(1).+rh)
----

The first group captures a word which is then scanned by the second group.
This example does not actually need this heavyweight feature; the same match
can be achieved with:

[source,regex]
----
\b\w+?rh\w*\b
----

When things are more complicated, however, scanning a captured substring can be
a useful way to describe the required match. For exmple, there is a rather
complicated pattern in the PCRE2 test data that checks an entire subject string
for a palindrome, that is, the sequence of letters is the same in both
directions. Suppose you want to search for individual words of two or more
characters such as "level" that are palindromes:

[source,regex]
----
(\b\w{2,}+\b)(*scs:(1)...palindrome-matching-pattern...)
----

Within a substring scanning subpattern, references to other groups work as
normal. Capturing groups may appear, and will retain their values during
ongoing matching if the assertion succeeds.

== Script runs

In concept, a script run is a sequence of characters that are all from the same
Unicode script such as Latin or Greek. However, because some scripts are
commonly used together, and because some diacritical and other marks are used
with multiple scripts, it is not that simple. There is a full description of
the rules that PCRE2 uses in the section
xref:pcre2unicode.adoc#script-runs[Script runs]
in the
xref:pcre2unicode.adoc[`pcre2unicode`]
documentation.

If part of a pattern is enclosed between [.regex]`+(*script_run:+` or [.regex]`+(*sr:+` and a closing
parenthesis, it fails if the sequence of characters that it matches are not a
script run. After a failure, normal backtracking occurs. Script runs can be
used to detect spoofing attacks using characters that look the same, but are
from different scripts. The string [.subject]`paypal.com` is an infamous example, where
the letters could be a mixture of Latin and Cyrillic. This pattern ensures that
the matched characters in a sequence of non-spaces that follow white space are
a script run:

[source,regex]
----
\s+(*sr:\S+)
----

To be sure that they are all from the Latin script (for example), a lookahead
can be used:

[source,regex]
----
\s+(?=\p{Latin})(*sr:\S+)
----

This works as long as the first character is expected to be a character in that
script, and not (for example) punctuation, which is allowed with any script. If
this is not the case, a more creative lookahead is needed. For example, if
digits, underscore, and dots are permitted at the start:

[source,regex]
----
\s+(?=[0-9_.]*\p{Latin})(*sr:\S+)
----

In many cases, backtracking into a script run pattern fragment is not
desirable. The script run can employ an atomic group to prevent this. Because
this is a common requirement, a shorthand notation is provided by
[.regex]`+(*atomic_script_run:+` or [.regex]`+(*asr:+`

[.regex]`+(*asr:...)+`::
    Is the same as [.regex]`+(*sr:(?>...))+`.

Note that the atomic group is inside the script run. Putting it outside would
not prevent backtracking into the script run pattern.

Support for script runs is not available if PCRE2 is compiled without Unicode
support. A compile-time error is given if any of the above constructs is
encountered. Script runs are not supported by the alternate matching function,
func:pcre2_dfa_match() because they use the same mechanism as capturing
parentheses.

Warning: The [.regex]`+(*ACCEPT)+` control verb
(see below)
should not be used within a script run group, because it causes an immediate
exit from the group, bypassing the script run checking.

[[conditional-groups]]
== Conditional groups

It is possible to cause the matching process to obey a pattern fragment
conditionally or to choose between two alternative fragments, depending on
the result of an assertion, or whether a specific capture group has
already been matched. The two possible forms of conditional group are:

[source,regex,subs=normal]
----
(?(__condition__)__yes-pattern__)
(?(__condition__)__yes-pattern__|__no-pattern__)
----

If the condition is satisfied, the yes-pattern is used; otherwise the
no-pattern (if present) is used. An absent no-pattern is equivalent to an empty
string (it always matches). If there are more than two alternatives in the
group, a compile-time error occurs. Each of the two alternatives may itself
contain nested groups of any form, including conditional groups; the
restriction to two alternatives applies only at the level of the condition
itself. This pattern fragment is an example where the alternatives are complex:

[source,regex]
----
(?(1) (A|B|C) | (D | (?(2)E|F) | E) )
----

There are five kinds of condition: references to capture groups, references to
recursion, two pseudo-conditions called DEFINE and VERSION, and assertions.

=== Checking for a used capture group by number

If the text between the parentheses consists of a sequence of digits, the
condition is true if a capture group of that number has previously matched. If
there is more than one capture group with the same number (see the earlier
section about duplicate group numbers),
the condition is true if any of them have matched. An alternative notation,
which is a PCRE2 extension, not supported by Perl, is to precede the digits
with a plus or minus sign. In this case, the group number is relative rather
than absolute. The most recently opened capture group (which could be enclosing
this condition) can be referenced by [.regex]`(?(-1)`, the next most recent by [.regex]`(?(-2)`,
and so on. Inside loops it can also make sense to refer to subsequent groups.
The next capture group to be opened can be referenced as [.regex]`(?(+1)`, and so on. The
value zero in any of these forms is not used; it provokes a compile-time error.

Consider the following pattern, which contains non-significant white space to
make it more readable (assume the const:PCRE2_EXTENDED option) and to divide it into
three parts for ease of discussion:

[source,regex]
----
( \( )?    [^()]+    (?(1) \) )
----

The first part matches an optional opening parenthesis, and if that
character is present, sets it as the first captured substring. The second part
matches one or more characters that are not parentheses. The third part is a
conditional group that tests whether or not the first capture group
matched. If it did, that is, if subject started with an opening parenthesis,
the condition is true, and so the yes-pattern is executed and a closing
parenthesis is required. Otherwise, since no-pattern is not present, the
conditional group matches nothing. In other words, this pattern matches a
sequence of non-parentheses, optionally enclosed in parentheses.

If you were embedding this pattern in a larger one, you could use a relative
reference:

[source,regex]
----
...other stuff... ( \( )?    [^()]+    (?(-1) \) ) ...
----

This makes the fragment independent of the parentheses in the larger pattern.

=== Checking for a used capture group by name

Perl uses the syntax [.regex]`(?(<__name__>)\...)` or [.regex]`(?('__name__')\...)` to test for a used
capture group by name. For compatibility with earlier versions of PCRE1, which
had this facility before Perl, the syntax [.regex]`(?(__name__)\...)` is also recognized.
Note, however, that undelimited names consisting of the letter R followed by
digits are ambiguous (see the section <<checking-pattern-recursion>>). Rewriting the above example
to use a named group gives this:

[source,regex]
----
(?<OPEN> \( )?    [^()]+    (?(<OPEN>) \) )
----

If the name used in a condition of this kind is a duplicate, the test is
applied to all groups of the same name, and is true if any one of them has
matched.

[[checking-pattern-recursion]]
=== Checking for pattern recursion

"Recursion" in this sense refers to any subroutine-like call from one part of
the pattern to another, whether or not it is actually recursive. See the
sections <<recursive-patterns>>
and
<<groups-as-subroutines>>
below for details of recursion and subroutine calls.

If a condition is the string [.regex]`+(R)+`, and there is no capture group with the name
R, the condition is true if matching is currently in a recursion or subroutine
call to the whole pattern or any capture group. If digits follow the letter R,
and there is no group with that name, the condition is true if the most recent
call is into a group with the given number, which must exist somewhere in the
overall pattern. This is a contrived example that is equivalent to [.regex]`a+b`:

[source,regex]
----
((?(R1)a+|(?1)b))
----

However, in both cases, if there is a capture group with a matching name, the
condition tests for its being set, as described in the section above, instead
of testing for recursion. For example, creating a group with the name R1 by
adding [.regex]`+(?<R1>)+` to the above pattern completely changes its meaning.

If a name preceded by ampersand follows the letter R, for example:

[source,regex,subs=normal]
----
(?(R&__name__)\...)
----

the condition is true if the most recent recursion is into a group of that name
(which must exist within the pattern).

This condition does not check the entire recursion stack. It tests only the
current level. If the name used in a condition of this kind is a duplicate, the
test is applied to all groups of the same name, and is true if any one of
them is the most recent recursion.

At "top level", all these recursion test conditions are false.

[[defining-capture-groups]]
=== Defining capture groups for use by reference only

If the condition is the string [.regex]`(DEFINE)`, the condition is always false, even if
there is a group with the name DEFINE. In this case, there may be only one
alternative in the rest of the conditional group. It is always skipped if
control reaches this point in the pattern; the idea of DEFINE is that it can be
used to define subroutines that can be referenced from elsewhere. (The use of
subroutines
is described below.) For example, a pattern to match an IPv4 address such as
[.subject]`192.168.23.245` could be written like this (ignore white space and line
breaks):

[source,regex]
----
(?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
\b (?&byte) (\.(?&byte)){3} \b
----

The first part of the pattern is a DEFINE group inside which another group
named "byte" is defined. This matches an individual component of an IPv4
address (a number less than 256). When matching takes place, this part of the
pattern is skipped because DEFINE acts like a false condition. The rest of the
pattern uses references to the named group to match the four dot-separated
components of an IPv4 address, insisting on a word boundary at each end.

=== Checking the PCRE2 version

Programs that link with a PCRE2 library can check the version by calling
func:pcre2_config() with appropriate arguments. Users of applications that do
not have access to the underlying code cannot do this. A special "condition"
called VERSION exists to allow such users to discover which version of PCRE2
they are dealing with by using this condition to match a string such as
[.subject]`yesno`. VERSION must be followed either by [.regex]`=` or [.regex]`>=` and a version number.
For example:

[source,regex]
----
(?(VERSION>=10.4)yes|no)
----

This pattern matches [.subject]`yes` if the PCRE2 version is greater or equal to 10.4, or
[.subject]`no` otherwise. The fractional part of the version number could be ommited.

=== Assertion conditions

If the condition is not in any of the above formats, it must be a parenthesized
assertion. This may be a positive or negative lookahead or lookbehind
assertion. However, it must be a traditional atomic assertion, not one of the
non-atomic assertions.

Consider this pattern, again containing non-significant white space, and with
the two alternatives on the second line:

[source,regex]
----
(?(?=[^a-z]*[a-z])
\d{2}-[a-z]{3}-\d{2}  |  \d{2}-\d{2}-\d{2} )
----

The condition is a positive lookahead assertion that matches an optional
sequence of non-letters followed by a letter. In other words, it tests for the
presence of at least one letter in the subject. If a letter is found, the
subject is matched against the first alternative; otherwise it is matched
against the second. This pattern matches strings in one of the two forms
[.subject]`_dd_-_aaa_-_dd_` or [.subject]`_dd_-_dd_-_dd_`, where _aaa_ are letters and _dd_ are digits.

When an assertion that is a condition contains capture groups, any
capturing that occurs in a matching branch is retained afterwards, for both
positive and negative assertions, because matching always continues after the
assertion, whether it succeeds or fails. (Compare non-conditional assertions,
for which captures are retained only for positive assertions that succeed.)

== Comments

There are two ways of including comments in patterns that are processed by
PCRE2. In both cases, the start of the comment must not be in a character
class, nor in the middle of any other sequence of related characters such as
[.regex]`(?:` or a group name or number or a Unicode property name. The characters that
make up a comment play no part in the pattern matching.

The sequence [.regex]`(?#` marks the start of a comment that continues up to the next
closing parenthesis. Nested parentheses are not permitted. If the
const:PCRE2_EXTENDED or const:PCRE2_EXTENDED_MORE option is set, an unescaped # character
also introduces a comment, which in this case continues to immediately after
the next newline character or character sequence in the pattern. Which
characters are interpreted as newlines is controlled by an option passed to the
compiling function or by a special sequence at the start of the pattern, as
described in the section <<newline-conventions>>
above. Note that the end of this type of comment is a literal newline sequence
in the pattern; escape sequences that happen to represent a newline do not
count. For example, consider this pattern when const:PCRE2_EXTENDED is set, and the
default newline convention (a single linefeed character) is in force:

[source,regex]
----
abc #comment \n still comment
----

On encountering the # character, func:pcre2_compile() skips along, looking for
a newline in the pattern. The sequence [.regex]`+\n+` is still literal at this stage, so
it does not terminate the comment. Only an actual character with the code value
char:0x0a (the default newline) does so.

[[recursive-patterns]]
== Recursive patterns

Consider the problem of matching a string in parentheses, allowing for
unlimited nested parentheses. Without the use of recursion, the best that can
be done is to use a pattern that matches up to some fixed depth of nesting. It
is not possible to handle an arbitrary nesting depth.

For some time, Perl has provided a facility that allows regular expressions to
recurse (amongst other things). It does this by interpolating Perl code in the
expression at run time, and the code can refer to the expression itself. A Perl
pattern using code interpolation to solve the parentheses problem can be
created like this:

[source,perl]
----
$re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;
----

The [.regex]`+(?p{...})+` item interpolates Perl code at run time, and in this case refers
recursively to the pattern in which it appears.

Obviously, PCRE2 cannot support the interpolation of Perl code. Instead, it
supports special syntax for recursion of the entire pattern, and also for
individual capture group recursion. After its introduction in PCRE1 and Python,
this kind of recursion was subsequently introduced into Perl at release 5.10.

A special item that consists of [.regex]`(?` followed by a number greater than zero and a
closing parenthesis is a recursive subroutine call of the capture group of the
given number, provided that it occurs inside that group. (If not, it is a
non-recursive subroutine
call, which is described in the section <<groups-as-subroutines>>.) The special item [.regex]`+(?R)+` or [.regex]`+(?0)+` is
a recursive call of the entire regular expression.

This PCRE2 pattern solves the nested parentheses problem (assume the
const:PCRE2_EXTENDED option is set so that white space is ignored):

[source,regex]
----
\( ( [^()]++ | (?R) )* \)
----

First it matches an opening parenthesis. Then it matches any number of
substrings which can either be a sequence of non-parentheses, or a recursive
match of the pattern itself (that is, a correctly parenthesized substring).
Finally there is a closing parenthesis. Note the use of a possessive quantifier
to avoid backtracking into sequences of non-parentheses.

If this were part of a larger pattern, you would not want to recurse the entire
pattern, so instead you could use this:

[source,regex]
----
( \( ( [^()]++ | (?1) )* \) )
----

We have put the pattern into parentheses, and caused the recursion to refer to
them instead of the whole pattern.

In a larger pattern, keeping track of parenthesis numbers can be tricky. This
is made easier by the use of relative references. Instead of [.regex]`(?1)` in the
pattern above you can write [.regex]`+(?-2)+` to refer to the second most recently opened
parentheses preceding the recursion. In other words, a negative number counts
capturing parentheses leftwards from the point at which it is encountered.

Be aware however, that if
duplicate capture group numbers
are in use, relative references refer to the earliest group with the
appropriate number. Consider, for example:

[source,regex]
----
(?|(a)|(b)) (c) (?-2)
----

The first two capture groups [.regex]`(a)` and [.regex]`(b)` are both numbered 1, and group [.regex]`(c)`
is number 2. When the reference [.regex]`+(?-2)+` is encountered, the second most recently
opened parentheses has the number 1, but it is the first such group (the [.regex]`(a)`
group) to which the recursion refers. This would be the same if an absolute
reference [.regex]`(?1)` was used. In other words, relative references are just a
shorthand for computing a group number.

It is also possible to refer to subsequent capture groups, by writing
references such as [.regex]`(?+2)`. However, these cannot be recursive because the
reference is not inside the parentheses that are referenced. They are always
non-recursive subroutine
calls, as described in the section <<groups-as-subroutines>>.

An alternative approach is to use named parentheses. The Perl syntax for this
is [.regex]`(?&__name__)`; PCRE1's earlier syntax [.regex]`(?P>__name__)` is also supported. We could
rewrite the above example as follows:

[source,regex]
----
(?<pn> \( ( [^()]++ | (?&pn) )* \) )
----

If there is more than one group with the same name, the earliest one is
used.

The example pattern that we have been looking at contains nested unlimited
repeats, and so the use of a possessive quantifier for matching strings of
non-parentheses is important when applying the pattern to strings that do not
match. For example, when this pattern is applied to

[source,regex]
----
(aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
----

it yields "no match" quickly. However, if a possessive quantifier is not used,
the match runs for a very long time indeed because there are so many different
ways the [.regex]`{plus}` and [.regex]`+*+` repeats can carve up the subject, and all have to be tested
before failure can be reported.

At the end of a match, the values of capturing parentheses are those from
the outermost level. If you want to obtain intermediate values, a callout
function can be used (see below and the
xref:pcre2callout.adoc[`pcre2callout`]
documentation). If the pattern above is matched against

[source,regex]
----
(ab(cd)ef)
----

the value for the inner capturing parentheses (numbered 2) is [.subject]`ef`, which is
the last value taken on at the top level. If a capture group is not matched at
the top level, its final captured value is unset, even if it was (temporarily)
set at a deeper level during the matching process.

Do not confuse the [.regex]`+(?R)+` item with the condition [.regex]`+(R)+`, which tests for recursion.
Consider this pattern, which matches text in angle brackets, allowing for
arbitrary nesting. Only digits are allowed in nested brackets (that is, when
recursing), whereas any characters are permitted at the outer level.

[source,regex]
----
< (?: (?(R) \d++  | [^<>]*+) | (?R)) * >
----

In this pattern, [.regex]`+(?(R)+` is the start of a conditional group, with two different
alternatives for the recursive and non-recursive cases. The [.regex]`+(?R)+` item is the
actual recursive call.

=== Differences in recursion processing between PCRE2 and Perl

Some former differences between PCRE2 and Perl no longer exist.

Before release 10.30, recursion processing in PCRE2 differed from Perl in that
a recursive subroutine call was always treated as an atomic group. That is,
once it had matched some of the subject string, it was never re-entered, even
if it contained untried alternatives and there was a subsequent matching
failure. (Historical note: PCRE implemented recursion before Perl did.)

Starting with release 10.30, recursive subroutine calls are no longer treated
as atomic. That is, they can be re-entered to try unused alternatives if there
is a matching failure later in the pattern. This is now compatible with the way
Perl works. If you want a subroutine call to be atomic, you must explicitly
enclose it in an atomic group.

Supporting backtracking into recursions simplifies certain types of recursive
pattern. For example, this pattern matches palindromic strings:

[source,regex]
----
^((.)(?1)\2|.?)$
----

The second branch in the group matches a single central character in the
palindrome when there are an odd number of characters, or nothing when there
are an even number of characters, but in order to work it has to be able to try
the second case when the rest of the pattern match fails. If you want to match
typical palindromic phrases, the pattern has to ignore all non-word characters,
which can be done like this:

[source,regex]
----
^\W*+((.)\W*+(?1)\W*+\2|\W*+.?)\W*+$
----

If run with the const:PCRE2_CASELESS option, this pattern matches phrases such as [.subject]`A
man, a plan, a canal: Panama!`. Note the use of the possessive quantifier [.regex]`*{plus}` to
avoid backtracking into sequences of non-word characters. Without this, PCRE2
takes a great deal longer (ten times or more) to match typical phrases, and
Perl takes so long that you think it has gone into a loop.

Another way in which PCRE2 and Perl used to differ in their recursion
processing is in the handling of captured values. Formerly in Perl, when a
group was called recursively or as a subroutine (see the section
<<groups-as-subroutines>>), it
had no access to any values that were captured outside the recursion, whereas
in PCRE2 these values can be referenced. Consider this pattern:

[source,regex]
----
^(.)(\1|a(?2))
----

This pattern matches [.subject]`bab`. The first capturing parentheses match [.subject]`b`, then in
the second group, when the backreference [.regex]`\1` fails to match [.subject]`b`, the second
alternative matches [.subject]`a` and then recurses. In the recursion, [.regex]`\1` does now match
[.subject]`b` and so the whole match succeeds. This match used to fail in Perl, but in
later versions (I tried 5.024) it now works.

[[groups-as-subroutines]]
=== Groups as subroutines

If the syntax for a recursive group call (either by number or by name) is used
outside the parentheses to which it refers, it operates a bit like a subroutine
in a programming language. More accurately, PCRE2 treats the referenced group
as an independent subpattern which it tries to match at the current matching
position. The called group may be defined before or after the reference. A
numbered reference can be absolute or relative, as in these examples:

[source,regex]
----
(...(absolute)...)...(?2)...
(...(relative)...)...(?-1)...
(...(?+1)...(relative)...
----

An earlier example pointed out that the pattern

[source,regex]
----
(sens|respons)e and \1ibility
----

matches [.subject]`sense and sensibility` and [.subject]`response and responsibility`, but not
[.subject]`sense and responsibility`. If instead the pattern

[source,regex]
----
(sens|respons)e and (?1)ibility
----

is used, it does match [.subject]`sense and responsibility` as well as the other two
strings. Another example is given in the discussion of DEFINE above.

Like recursions, subroutine calls used to be treated as atomic, but this
changed at PCRE2 release 10.30, so backtracking into subroutine calls can now
occur. However, any capturing parentheses that are set during the subroutine
call revert to their previous values afterwards.

Processing options such as case-independence are fixed when a group is
defined, so if it is used as a subroutine, such options cannot be changed for
different calls. For example, consider this pattern:

[source,regex]
----
(abc)(?i:(?-1))
----

It matches [.subject]`abcabc`. It does not match [.subject]`abcABC` because the change of
processing option does not affect the called group.

The behaviour of
backtracking control verbs
in groups when called as subroutines is described in the section
<<backtracking-verbs-in-subroutines>>
below.

=== Recursion and subroutines with returned capture groups

Since PCRE2 10.47, recursion and subroutine calls may also specify a list of
capture groups to return. This is a PCRE2 syntax extension not supported by
Perl. The pattern matching recurses into the referenced expression as described
above, however, when the recursion returns to the calling expression the
subgroups captured during the recursion can be retained when the calling
expression's context is restored.

When used as a subroutine, this allows the subroutine's capture groups to
be used as return values.

Only the specific capture groups listed by the caller will be retained, using
the following syntax:

[.regex]`(?R(_grouplist_))`::
[.regex]`(?_n_(_grouplist_))`::
[.regex]`(?+_n_(_grouplist_))`::
    Recurse the whole pattern, returning capture groups.
[.regex]`(?-_n_(_grouplist_))`::
[.regex]`(?&__name__(_grouplist_))`::
[.regex]`(?P>__name__(_grouplist_))`::
    Call a subroutine, returning capture groups.

The list of capture groups _grouplist_ is a comma-separated list of (absolute
or relative) group numbers, and group names enclosed in single quotes or angle
brackets.

Here is an example which first uses the DEFINE condition to create a re-usable
routine for matching a weekday, then calls that subroutine and retains the
groups it captures for use later:

[source,regex]
----
(?x: # ignore whitespace for clarity
  # Define the routine "weekendday" which matches Saturday or
  # Sunday, and returns the Sat/Sun prefix as \k<short>.
  (?(DEFINE) (?<weekendday>
      (?|(?<short>Sat)urday|(?<short>Sun)day) ) )
  # Call the routine. Matches "Saturday,Sat" or "Sunday,Sun".
  (?&weekendday(<short>)),\k<short> )
----

This feature is not available using the Oniguruma syntax [.regex]`+\g<...>+` or [.regex]`+\g'...'+`
below.

=== Oniguruma subroutine syntax

For compatibility with Oniguruma, the non-Perl syntax [.regex]`+\g+` followed by a name or
a number enclosed either in angle brackets or single quotes, is an alternative
syntax for calling a group as a subroutine, possibly recursively. Here are two
of the examples used above, rewritten using this syntax:

[source,regex]
----
(?<pn> \( ( (?>[^()]+) | \g<pn> )* \) )
(sens|respons)e and \g'1'ibility
----

PCRE2 supports an extension to Oniguruma: if a number is preceded by a
plus or a minus sign it is taken as a relative reference. For example:

[source,regex]
----
(abc)(?i:\g<-1>)
----

Note that [.regex]`+\g{...}+` (Perl syntax) and [.regex]`+\g<...>+` (Oniguruma syntax) are _not_
synonymous. The former is a backreference; the latter is a subroutine call.

== Callouts

Perl has a feature whereby using the sequence [.regex]`+(?{...})+` causes arbitrary Perl
code to be obeyed in the middle of matching a regular expression. This makes it
possible, amongst other things, to extract different substrings that match the
same pair of parentheses when there is a repetition.

PCRE2 provides a similar feature, but of course it cannot obey arbitrary Perl
code. The feature is called "callout". The caller of PCRE2 provides an external
function by putting its entry point in a match context using the function
func:pcre2_set_callout(), and then passing that context to func:pcre2_match()
or func:pcre2_dfa_match(). If no match context is passed, or if the callout
entry point is set to NULL, callout points will be passed over silently during
matching. To disallow callouts in the pattern syntax, you may use the
const:PCRE2_EXTRA_NEVER_CALLOUT option.

Within a regular expression, [.regex]`(?C__arg__)` indicates a point at which the external
function is to be called. There are two kinds of callout: those with a
numerical argument and those with a string argument. [.regex]`+(?C)+` on its own with no
argument is treated as [.regex]`+(?C0)+`. A numerical argument allows the application to
distinguish between different callouts. String arguments were added for release
10.20 to make it possible for script languages that use PCRE2 to embed short
scripts within patterns in a similar way to Perl.

During matching, when PCRE2 reaches a callout point, the external function is
called. It is provided with the number or string argument of the callout, the
position in the pattern, and one item of data that is also set in the match
block. The callout function may cause matching to proceed, to backtrack, or to
fail.

By default, PCRE2 implements a number of optimizations at matching time, and
one side-effect is that sometimes callouts are skipped. If you need all
possible callouts to happen, you need to set options that disable the relevant
optimizations. More details, including a complete description of the
programming interface to the callout function, are given in the
xref:pcre2callout.adoc[`pcre2callout`]
documentation.

=== Callouts with numerical arguments

If you just want to have a means of identifying different callout points, put a
number no greater than macroval:MAX_CALLOUT_NUMBER after the letter C. For example, this pattern has two
callout points:

[source,regex]
----
(?C1)abc(?C2)def
----

If the const:PCRE2_AUTO_CALLOUT flag is passed to func:pcre2_compile(), numerical
callouts are automatically installed before each item in the pattern. They are
all numbered macroval:MAX_CALLOUT_NUMBER. If there is a conditional group in the pattern whose
condition is an assertion, an additional callout is inserted just before the
condition. An explicit callout may also be set at this position, as in this
example:

[source,regex]
----
(?(?C9)(?=a)abc|def)
----

Note that this applies only to assertion conditions, not to other types of
condition.

=== Callouts with string arguments

A delimited string may be used instead of a number as a callout argument. The
starting delimiter must be one of [.regex]`{backtick}`, [.regex]`'`, [.regex]`"`, [.regex]`^`, [.regex]`%`, [.regex]`#`, [.regex]`$` or [.regex]`{` and the ending delimiter is
the same as the start, except for [.regex]`{`, where the ending delimiter is [.regex]`}`. If the
ending delimiter is needed within the string, it must be doubled. For
example:

[source,regex]
----
(?C'ab ''c'' d')xyz(?C{any text})pqr
----

The doubling is removed before the string is passed to the callout function.

== Backtracking control

There are a number of special "Backtracking Control Verbs" (to use Perl's
terminology) that modify the behaviour of backtracking during matching. They
are generally of the form [.regex]`+(*VERB)+` or [.regex]`(*VERB:__NAME__)`. Some verbs take either form,
and may behave differently depending on whether or not a name argument is
present. The names are not required to be unique within the pattern.

By default, for compatibility with Perl, a name is any sequence of characters
that does not include a closing parenthesis. The name is not processed in
any way, and it is not possible to include a closing parenthesis in the name.
This can be changed by setting the const:PCRE2_ALT_VERBNAMES option, but the result
is no longer Perl-compatible.

When const:PCRE2_ALT_VERBNAMES is set, backslash processing is applied to verb names
and only an unescaped closing parenthesis terminates the name. However, the
only backslash items that are permitted are [.regex]`+\Q+`, [.regex]`+\E+`, and sequences such as
[.regex]`+\x{100}+` that define character code points. Character type escapes such as [.regex]`\d`
are faulted.

A closing parenthesis can be included in a name either as [.regex]`+\)+` or between [.regex]`+\Q+`
and [.regex]`+\E+`. In addition to backslash processing, if the const:PCRE2_EXTENDED or
const:PCRE2_EXTENDED_MORE option is also set, unescaped white space in verb names is
skipped, and #-comments are recognized, exactly as in the rest of the pattern.
const:PCRE2_EXTENDED and const:PCRE2_EXTENDED_MORE do not affect verb names unless
const:PCRE2_ALT_VERBNAMES is also set.

The maximum length of a name is macroval:MAX_MARK_8 in the 8-bit library and macroval:MAX_MARK_16_32 in the
16-bit and 32-bit libraries. If the name is empty, that is, if the closing
parenthesis immediately follows the colon, the effect is as if the colon were
not there. Any number of these verbs may occur in a pattern. Except for
[.regex]`+(*ACCEPT)+`, they may not be quantified.

Since these verbs are specifically related to backtracking, most of them can be
used only when the pattern is to be matched using the traditional matching
function or JIT, because they use backtracking algorithms. With the exception
of [.regex]`+(*FAIL)+`, which behaves like a failing negative assertion, the backtracking
control verbs cause an error if encountered by the DFA matching function.

The behaviour of these verbs in
repeated groups,
assertions,
and in
capture groups called as subroutines
(whether or not recursively) is documented below.

=== Optimizations that affect backtracking verbs

PCRE2 contains some optimizations that are used to speed up matching by running
some checks at the start of each match attempt. For example, it may know the
minimum length of matching subject, or that a particular character must be
present. When one of these optimizations bypasses the running of a match, any
included backtracking verbs will not, of course, be processed. You can suppress
the start-of-match optimizations by setting the const:PCRE2_NO_START_OPTIMIZE option
when calling func:pcre2_compile(), by calling func:pcre2_set_optimize() with a
const:PCRE2_START_OPTIMIZE_OFF directive, or by starting the pattern with
[.regex]`+(*NO_START_OPT)+`. There is more discussion of this option in the section
xref:pcre2api.adoc#compiling-a-pattern[Compiling a pattern]
in the
xref:pcre2api.adoc[`pcre2api`]
documentation.

Experiments with Perl suggest that it too has similar optimizations, and like
PCRE2, turning them off can change the result of a match.

=== Verbs that act immediately

The following verbs act as soon as they are encountered.

[.regex]`(*ACCEPT)` or [.regex]`(*ACCEPT:__NAME__)`::

This verb causes the match to end successfully, skipping the remainder of the
pattern. However, when it is inside a capture group that is called as a
subroutine, only that group is ended successfully. Matching then continues
at the outer level. If [.regex]`+(*ACCEPT)+` in triggered in a positive assertion, the
assertion succeeds; in a negative assertion, the assertion fails.
+
If [.regex]`+(*ACCEPT)+` is inside capturing parentheses, the data so far is captured. For
example:
+
[source,regex]
----
A((?:A|B(*ACCEPT)|C)D)
----
+
This matches [.subject]`AB`, [.subject]`AAD`, or [.subject]`ACD`; when it matches [.subject]`AB`, [.subject]`B` is captured by
the outer parentheses.
+
[.regex]`+(*ACCEPT)+` is the only backtracking verb that is allowed to be quantified
because an ungreedy quantification with a minimum of zero acts only when a
backtrack happens. Consider, for example,
+
[source,regex]
----
(A(*ACCEPT)??B)C
----
+
where A, B, and C may be complex expressions. After matching [.subject]`A`, the matcher
processes [.subject]`BC`; if that fails, causing a backtrack, [.regex]`+(*ACCEPT)+` is triggered and
the match succeeds. In both cases, all but C is captured. Whereas [.regex]`+(*COMMIT)+`
(see below) means "fail on backtrack", a repeated [.regex]`+(*ACCEPT)+` of this type means
"succeed on backtrack".
+
Warning: [.regex]`+(*ACCEPT)+` should not be used within a script run group, because
it causes an immediate exit from the group, bypassing the script run checking.

[.regex]`(*FAIL)` or [.regex]`(*FAIL:__NAME__)`::

This verb causes a matching failure, forcing backtracking to occur. It may be
abbreviated to [.regex]`+(*F)+`. It is equivalent to [.regex]`+(?!)+` but easier to read. The Perl
documentation notes that it is probably useful only when combined with [.regex]`(?{})` or
[.regex]`(??{})`. Those are, of course, Perl features that are not present in PCRE2. The
nearest equivalent is the callout feature, as for example in this pattern:
+
[source,regex]
----
a+(?C)(*FAIL)
----
+
A match with the string [.subject]`aaaa` always fails, but the callout is taken before
each backtrack happens (in this example, 10 times).

[.regex]`(*ACCEPT:__NAME__)` and [.regex]`(*FAIL:__NAME__)` behave the same as [.regex]`(*MARK:__NAME__)(*ACCEPT)` and
[.regex]`(*MARK:__NAME__)(*FAIL)`, respectively, that is, a [.regex]`+(*MARK)+` is recorded just before
the verb acts.

=== Recording which path was taken

There is one verb whose main purpose is to track how a match was arrived at,
though it also has a secondary use in conjunction with advancing the match
starting point (see [.regex]`+(*SKIP)+` below).

[.regex]`(*MARK:__NAME__)` or [.regex]`({asterisk}:__NAME__)`::

A name is always required with this verb. For all the other backtracking
control verbs, a _NAME_ argument is optional.
+
When a match succeeds, the name of the last-encountered mark name on the
matching path is passed back to the caller as described in the section
xref:pcre2api.adoc#other-match-information[Other information about a match]
in
xref:pcre2api.adoc[`pcre2api`]
documentation. This applies to all instances of [.regex]`+(*MARK)+` and other verbs,
including those inside assertions and atomic groups. However, there are
differences in those cases when [.regex]`+(*MARK)+` is used in conjunction with [.regex]`+(*SKIP)+` as
described below.
+
The mark name that was last encountered on the matching path is passed back. A
verb without a _NAME_ argument is ignored for this purpose. Here is an example of
`pcre2test` output, where the "mark" modifier requests the retrieval and
outputting of [.regex]`+(*MARK)+` data:
+
[source,text]
----
  re> /X(*MARK:A)Y|X(*MARK:B)Z/mark
data> XY
 0: XY
MK: A
XZ
 0: XZ
MK: B
----
+
The [.regex]`+(*MARK)+` name is tagged with "MK:" in this output, and in this example it
indicates which of the two alternatives matched. This is a more efficient way
of obtaining this information than putting each alternative in its own
capturing parentheses.
+
If a verb with a name is encountered in a positive assertion that is true, the
name is recorded and passed back if it is the last-encountered. This does not
happen for negative assertions or failing positive assertions.
+
After a partial match or a failed match, the last encountered name in the
entire match process is returned. For example:
+
[source,text]
----
  re> /X(*MARK:A)Y|X(*MARK:B)Z/mark
data> XP
No match, mark = B
----
+
Note that in this unanchored example the mark is retained from the match
attempt that started at the letter "X" in the subject. Subsequent match
attempts starting at "P" and then with an empty string do not get as far as the
[.regex]`+(*MARK)+` item, but nevertheless do not reset it.
+
If you are interested in [.regex]`+(*MARK)+` values after failed matches, you should
probably either set the const:PCRE2_NO_START_OPTIMIZE option or call
func:pcre2_set_optimize() with a const:PCRE2_START_OPTIMIZE_OFF directive
(see above)
to ensure that the match is always attempted.

=== Verbs that act after backtracking

The following verbs do nothing when they are encountered. Matching continues
with what follows, but if there is a subsequent match failure, causing a
backtrack to the verb, a failure is forced. That is, backtracking cannot pass
to the left of the verb. However, when one of these verbs appears inside an
atomic group or in an atomic lookaround assertion that is true, its effect is
confined to that group, because once the group has been matched, there is never
any backtracking into it. Backtracking from beyond an atomic assertion or group
ignores the entire group, and seeks a preceding backtracking point.

These verbs differ in exactly what kind of failure occurs when backtracking
reaches them. The behaviour described below is what happens when the verb is
not in a subroutine or an assertion. Subsequent sections cover these special
cases.

[.regex]`(*COMMIT)` or [.regex]`(*COMMIT:__NAME__)`::

This verb causes the whole match to fail outright if there is a later matching
failure that causes backtracking to reach it. Even if the pattern is
unanchored, no further attempts to find a match by advancing the starting point
take place. If [.regex]`+(*COMMIT)+` is the only backtracking verb that is encountered,
once it has been passed func:pcre2_match() is committed to finding a match at
the current starting point, or not at all. For example:
+
[source,regex]
----
a+(*COMMIT)b
----
+
This matches [.subject]`xxaab` but not [.subject]`aacaab`. It can be thought of as a kind of
dynamic anchor, or "I've started, so I must finish."
+
The behaviour of [.regex]`(*COMMIT:__NAME__)` is not the same as [.regex]`(*MARK:__NAME__)(*COMMIT)`. It is
like [.regex]`(*MARK:__NAME__)` in that the name is remembered for passing back to the
caller. However, [.regex]`(*SKIP:__NAME__)` searches only for names that are set with
[.regex]`(*MARK)`, ignoring those set by any of the other backtracking verbs.
+
If there is more than one backtracking verb in a pattern, a different one that
follows [.regex]`+(*COMMIT)+` may be triggered first, so merely passing [.regex]`+(*COMMIT)+` during a
match does not always guarantee that a match must be at this starting point.
+
Note that [.regex]`+(*COMMIT)+` at the start of a pattern is not the same as an anchor,
unless PCRE2's start-of-match optimizations are turned off, as shown in this
output from `pcre2test`:
+
[source,text]
----
  re> /(*COMMIT)abc/
data> xyzabc
 0: abc
data>
re> /(*COMMIT)abc/no_start_optimize
data> xyzabc
No match
----
+
For the first pattern, PCRE2 knows that any match must start with [.subject]`a`, so the
optimization skips along the subject to [.subject]`a` before applying the pattern to the
first set of data. The match attempt then succeeds. The second pattern disables
the optimization that skips along to the first character. The pattern is now
applied starting at [.subject]`x`, and so the [.regex]`+(*COMMIT)+` causes the match to fail without
trying any other starting points.

[.regex]`+(*PRUNE)+` or [.regex]`(*PRUNE:__NAME__)`::

This verb causes the match to fail at the current starting position in the
subject if there is a later matching failure that causes backtracking to reach
it. If the pattern is unanchored, the normal "bumpalong" advance to the next
starting character then happens. Backtracking can occur as usual to the left of
[.regex]`+(*PRUNE)+`, before it is reached, or when matching to the right of [.regex]`+(*PRUNE)+`, but
if there is no match to the right, backtracking cannot cross [.regex]`+(*PRUNE)+`. In
simple cases, the use of [.regex]`+(*PRUNE)+` is just an alternative to an atomic group or
possessive quantifier, but there are some uses of [.regex]`+(*PRUNE)+` that cannot be
expressed in any other way. In an anchored pattern [.regex]`+(*PRUNE)+` has the same effect
as [.regex]`+(*COMMIT)+`.
+
The behaviour of [.regex]`(*PRUNE:__NAME__)` is not the same as [.regex]`(*MARK:__NAME__)(*PRUNE)`. It is
like [.regex]`(*MARK:__NAME__)` in that the name is remembered for passing back to the
caller. However, [.regex]`(*SKIP:__NAME__)` searches only for names set with [.regex]`(*MARK)`,
ignoring those set by other backtracking verbs.

[.regex]`(*SKIP)`::

This verb, when given without a name, is like [.regex]`+(*PRUNE)+`, except that if the
pattern is unanchored, the "bumpalong" advance is not to the next character,
but to the position in the subject where [.regex]`+(*SKIP)+` was encountered. [.regex]`+(*SKIP)+`
signifies that whatever text was matched leading up to it cannot be part of a
successful match if there is a later mismatch. Consider:
+
[source,regex]
----
a+(*SKIP)b
----
+
If the subject is [.subject]`aaaac...`, after the first match attempt fails (starting at
the first character in the string), the starting point skips on to start the
next attempt at [.subject]`c`. Note that a possessive quantifier does not have the same
effect as this example; although it would suppress backtracking during the
first match attempt, the second attempt would start at the second character
instead of skipping on to "c".
+
If [.regex]`+(*SKIP)+` is used to specify a new starting position that is the same as the
starting position of the current match, or (by being inside a lookbehind)
earlier, the position specified by [.regex]`+(*SKIP)+` is ignored, and instead the normal
"bumpalong" occurs.

[.regex]`(*SKIP:__NAME__)`::

When [.regex]`+(*SKIP)+` has an associated name, its behaviour is modified. When such a
[.regex]`+(*SKIP)+` is triggered, the previous path through the pattern is searched for the
most recent [.regex]`+(*MARK)+` that has the same name. If one is found, the "bumpalong"
advance is to the subject position that corresponds to that [.regex]`+(*MARK)+` instead of
to where [.regex]`+(*SKIP)+` was encountered. If no [.regex]`+(*MARK)+` with a matching name is found,
the [.regex]`+(*SKIP)+` is ignored.
+
The search for a [.regex]`+(*MARK)+` name uses the normal backtracking mechanism, which
means that it does not see [.regex]`+(*MARK)+` settings that are inside atomic groups or
assertions, because they are never re-entered by backtracking. Compare the
following `pcre2test` examples:
+
[source,text]
----
  re> /a(?>(*MARK:X))(*SKIP:X)(*F)|(.)/
data: abc
 0: a
 1: a
data:
  re> /a(?:(*MARK:X))(*SKIP:X)(*F)|(.)/
data: abc
 0: b
 1: b
----
+
In the first example, the [.regex]`+(*MARK)+` setting is in an atomic group, so it is not
seen when [.regex]`+(*SKIP:X)+` triggers, causing the [.regex]`+(*SKIP)+` to be ignored. This allows
the second branch of the pattern to be tried at the first character position.
In the second example, the [.regex]`+(*MARK)+` setting is not in an atomic group. This
allows [.regex]`+(*SKIP:X)+` to find the [.regex]`+(*MARK)+` when it backtracks, and this causes a new
matching attempt to start at the second character. This time, the [.regex]`+(*MARK)+` is
never seen because [.subject]`a` does not match [.subject]`b`, so the matcher immediately jumps to
the second branch of the pattern.
+
Note that [.regex]`(*SKIP:__NAME__)` searches only for names set by [.regex]`(*MARK:__NAME__)`. It ignores
names that are set by other backtracking verbs.

[.regex]`(*THEN)` or [.regex]`(*THEN:__NAME__)`::

This verb causes a skip to the next innermost alternative when backtracking
reaches it. That is, it cancels any further backtracking within the current
alternative. Its name comes from the observation that it can be used for a
pattern-based if-then-else block:
+
[source,regex]
----
( COND1 (*THEN) FOO |
  COND2 (*THEN) BAR |
  COND3 (*THEN) BAZ ) ...
----
+
If the COND1 pattern matches, FOO is tried (and possibly further items after
the end of the group if FOO succeeds); on failure, the matcher skips to the
second alternative and tries COND2, without backtracking into COND1. If that
succeeds and BAR fails, COND3 is tried. If subsequently BAZ fails, there are no
more alternatives, so there is a backtrack to whatever came before the entire
group. If [.regex]`+(*THEN)+` is not inside an alternation, it acts like [.regex]`+(*PRUNE)+`.
+
The behaviour of [.regex]`(*THEN:__NAME__)` is not the same as [.regex]`(*MARK:__NAME__)(*THEN)`. It is
like [.regex]`(*MARK:__NAME__)` in that the name is remembered for passing back to the
caller. However, [.regex]`(*SKIP:__NAME__)` searches only for names set with [.regex]`+(*MARK)+`,
ignoring those set by other backtracking verbs.
+
A group that does not contain a | character is just a part of the enclosing
alternative; it is not a nested alternation with only one alternative. The
effect of [.regex]`+(*THEN)+` extends beyond such a group to the enclosing alternative.
Consider this pattern, where A, B, etc. are complex pattern fragments that do
not contain any | characters at this level:
+
[source,regex]
----
A (B(*THEN)C) | D
----
+
If A and B are matched, but there is a failure in C, matching does not
backtrack into A; instead it moves to the next alternative, that is, D.
However, if the group containing [.regex]`+(*THEN)+` is given an alternative, it
behaves differently:
+
[source,regex]
----
A (B(*THEN)C | (*FAIL)) | D
----
+
The effect of [.regex]`+(*THEN)+` is now confined to the inner group. After a failure in C,
matching moves to [.regex]`+(*FAIL)+`, which causes the whole group to fail because there
are no more alternatives to try. In this case, matching does backtrack into A.
+
Note that a conditional group is not considered as having two alternatives,
because only one is ever used. In other words, the | character in a conditional
group has a different meaning. Ignoring white space, consider:
+
[source,regex]
----
^.*? (?(?=a) a | b(*THEN)c )
----
+
If the subject is [.subject]`ba`, this pattern does not match. Because [.regex]`+.*?+` is ungreedy,
it initially matches zero characters. The condition [.regex]`+(?=a)+` then fails, the
character [.subject]`b` is matched, but [.subject]`c` is not. At this point, matching does not
backtrack to [.regex]`+.*?+` as might perhaps be expected from the presence of the |
character. The conditional group is part of the single alternative that
comprises the whole pattern, and so the match fails. (If there was a backtrack
into [.regex]`+.*?+`, allowing it to match [.subject]`b`, the match would succeed.)

The verbs just described provide four different "strengths" of control when
subsequent matching fails. [.regex]`+(*THEN)+` is the weakest, carrying on the match at the
next alternative. [.regex]`+(*PRUNE)+` comes next, failing the match at the current
starting position, but allowing an advance to the next character (for an
unanchored pattern). [.regex]`+(*SKIP)+` is similar, except that the advance may be more
than one character. [.regex]`+(*COMMIT)+` is the strongest, causing the entire match to
fail.

=== More than one backtracking verb

If more than one backtracking verb is present in a pattern, the one that is
backtracked onto first acts. For example, consider this pattern, where A, B,
etc. are complex pattern fragments:

[source,regex]
----
(A(*COMMIT)B(*THEN)C|ABD)
----

If A matches but B fails, the backtrack to [.regex]`+(*COMMIT)+` causes the entire match to
fail. However, if A and B match, but C fails, the backtrack to [.regex]`+(*THEN)+` causes
the next alternative (ABD) to be tried. This behaviour is consistent, but is
not always the same as Perl's. It means that if two or more backtracking verbs
appear in succession, all but the last of them has no effect. Consider this
example:

[source,regex]
----
...(*COMMIT)(*PRUNE)...
----

If there is a matching failure to the right, backtracking onto [.regex]`+(*PRUNE)+` causes
it to be triggered, and its action is taken. There can never be a backtrack
onto [.regex]`+(*COMMIT)+`.

=== Backtracking verbs in repeated groups

PCRE2 sometimes differs from Perl in its handling of backtracking verbs in
repeated groups. For example, consider:

[source,regex]
----
(a(*COMMIT)b)+ac
----

If the subject is [.subject]`abac`, Perl matches unless its optimizations are disabled,
but PCRE2 always fails because the [.regex]`+(*COMMIT)+` in the second repeat of the group
acts.

=== Backtracking verbs in assertions

[.regex]`+(*FAIL)+` in any assertion has its normal effect: it forces an immediate
backtrack. The behaviour of the other backtracking verbs depends on whether or
not the assertion is standalone or acting as the condition in a conditional
group.

[.regex]`+(*ACCEPT)+` in a standalone positive assertion causes the assertion to succeed
without any further processing; captured strings and a mark name (if set) are
retained. In a standalone negative assertion, [.regex]`+(*ACCEPT)+` causes the assertion to
fail without any further processing; captured substrings and any mark name are
discarded.

If the assertion is a condition, [.regex]`+(*ACCEPT)+` causes the condition to be true for
a positive assertion and false for a negative one; captured substrings are
retained in both cases.

The remaining verbs act only when a later failure causes a backtrack to
reach them. This means that, for the Perl-compatible assertions, their effect
is confined to the assertion, because Perl lookaround assertions are atomic. A
backtrack that occurs after such an assertion is complete does not jump back
into the assertion. Note in particular that a [.regex]`+(*MARK)+` name that is set in an
assertion is not "seen" by an instance of [.regex]`(*SKIP:__NAME__)` later in the pattern.

PCRE2 now supports non-atomic positive assertions and also "scan substring"
assertions, as described in the sections <<non-atomic-assertions>>
and
<<scan-substring-assertions>>
above. These assertions must be standalone (not used as conditions). They are
not Perl-compatible. For these assertions, a later backtrack does jump back
into the assertion, and therefore verbs such as [.regex]`+(*COMMIT)+` can be triggered by
backtracks from later in the pattern.

The effect of [.regex]`+(*THEN)+` is not allowed to escape beyond an assertion. If there
are no more branches to try, [.regex]`+(*THEN)+` causes a positive assertion to be false,
and a negative assertion to be true. This behaviour differs from Perl when the
assertion has only one branch.

The other backtracking verbs are not treated specially if they appear in a
standalone positive assertion. In a conditional positive assertion,
backtracking (from within the assertion) into [.regex]`+(*COMMIT)+`, [.regex]`+(*SKIP)+`, or [.regex]`+(*PRUNE)+`
causes the condition to be false. However, for both standalone and conditional
negative assertions, backtracking into [.regex]`+(*COMMIT)+`, [.regex]`+(*SKIP)+`, or [.regex]`+(*PRUNE)+` causes
the assertion to be true, without considering any further alternative branches.

[[backtracking-verbs-in-subroutines]]
=== Backtracking verbs in subroutines

These behaviours occur whether or not the group is called recursively.

[.regex]`+(*ACCEPT)+` in a group called as a subroutine causes the subroutine match to
succeed without any further processing. Matching then continues after the
subroutine call. Perl documents this behaviour. Perl's treatment of the other
verbs in subroutines is different in some cases.

[.regex]`+(*FAIL)+` in a group called as a subroutine has its normal effect: it forces
an immediate backtrack.

[.regex]`+(*COMMIT)+`, [.regex]`+(*SKIP)+`, and [.regex]`+(*PRUNE)+` cause the subroutine match to fail when
triggered by being backtracked to in a group called as a subroutine. There is
then a backtrack at the outer level.

[.regex]`+(*THEN)+`, when triggered, skips to the next alternative in the innermost
enclosing group that has alternatives (its normal behaviour). However, if there
is no such group within the subroutine's group, the subroutine match fails and
there is a backtrack at the outer level.

[[ebcdic-environments]]
== EBCDIC environments

Differences in the way PCRE behaves when it is running in an EBCDIC environment
are covered in this section.

=== Escape sequences

When PCRE2 is compiled in EBCDIC mode, [.regex]`\N{U+__hhh\...__}` is not supported. [.regex]`+\a+`, [.regex]`+\e+`,
[.regex]`+\f+`, [.regex]`+\n+`, [.regex]`+\r+`, and [.regex]`+\t+` generate the appropriate EBCDIC code values. The [.regex]`+\c+`
escape is processed as specified for Perl in the `perlebcdic` document. The
only characters that are allowed after [.regex]`+\c+` are A--Z, a--z, or one of @, [, \, ],
^, _, or ?. Any other character provokes a compile-time error. The sequence
[.regex]`+\c@+` encodes character code 0; after [.regex]`+\c+` the letters (in either case) encode
characters 1--26 (char:0x01 to char:0x1A); [, \, ], ^, and _ encode characters 27--31
(char:0x1B to char:0x1F), and [.regex]`+\c?+` becomes either 255 (char:0xFF) or 95 (char:0x5F).

Thus, apart from [.regex]`+\c?+`, these escapes generate the same character code values as
they do in an ASCII or Unicode environment, though the meanings of the values
mostly differ. For example, [.regex]`+\cG+` always generates code value 7, which is BEL in
ASCII but DEL in EBCDIC.

The sequence [.regex]`+\c?+` generates DEL (127, char:0x7F) in an ASCII environment, but
because 127 is not a control character in EBCDIC, Perl makes it generate the
APC character. Unfortunately, there are several variants of EBCDIC. In most of
them the APC character has the value 255 (char:0xFF), but in the one Perl calls
POSIX-BC its value is 95 (char:0x5F). If certain other characters have POSIX-BC
values, PCRE2 makes [.regex]`+\c?+` generate 95; otherwise it generates 255.

=== Character classes

In character classes there is a special case in EBCDIC environments for ranges
whose end points are both specified as literal letters in the same case. For
compatibility with Perl, EBCDIC code points within the range that are not
letters are omitted. For example, [.regex]`[h-k]` matches only four characters, even
though the EBCDIC codes for h and k are char:0x88 and char:0x92, a range of 11 code
points. However, if the range is specified numerically, for example,
[.regex]`[\x88-\x92]` or [.regex]`[h-\x92]`, all code points are included.

== See also

xref:pcre2api.adoc[`pcre2api`], xref:pcre2callout.adoc[`pcre2callout`], xref:pcre2matching.adoc[`pcre2matching`],
xref:pcre2syntax.adoc[`pcre2syntax`], xref:pcre2.adoc[`pcre2`].

[discrete]
== Author

Philip Hazel +
Retired from University Computing Service +
Cambridge, England.

[discrete]
== Revision

Last updated: 25 October 2025 +
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