Test for *sub-replace-special* and *sub-replace-expression* on :substitute.
Test for submatch() on :substitue.
Test for *:s%* on :substitute.

STARTTEST
:so small.vim
ENDTEST

Results of test71:

STARTTEST
:set magic
:set cpo&
/^TEST/
j:s/A/&&/
j:s/B/\&/
j:s/C\(.\)\(.\)\(.\)\(.\)\(.\)\(.\)\(.\)\(.\)\(.\)/\0\9\8\7\6\5\4\3\2\1/
j:s/D/d/
j:s/E/~/
j:s/F/\~/
j:s/G/\ugg/
j:s/H/\Uh\Eh/
j:s/I/\lII/
j:s/J/\LJ\EJ/
j:s/K/\Uk\ek/
j:s/L//
j:s/M/\r/
j:s/N/\/
j:s/O/\n/
j:s/P/\b/
j:s/Q/\t/
j:s/R/\\/
j:s/S/\c/
j:s/T/ /
ENDTEST

TEST_1:
A
B
C123456789
D
E
F
G
H
I
J
K
lLl
mMm
nNn
oOo
pPp
qQq
rRr
sSs
tTt

STARTTEST
:set nomagic
:set cpo&
/^TEST/
j:s/A/&&/
j:s/B/\&/
j:s/\mC\(.\)\(.\)\(.\)\(.\)\(.\)\(.\)\(.\)\(.\)\(.\)/\0\9\8\7\6\5\4\3\2\1/
j:s/D/d/
j:s/E/~/
j:s/F/\~/
j:s/G/\ugg/
j:s/H/\Uh\Eh/
j:s/I/\lII/
j:s/J/\LJ\EJ/
j:s/K/\Uk\ek/
j:s/L//
j:s/M/\r/
j:s/N/\/
j:s/O/\n/
j:s/P/\b/
j:s/Q/\t/
j:s/R/\\/
j:s/S/\c/
j:s/T/ /
ENDTEST

TEST_2:
A
B
C123456789
D
E
F
G
H
I
J
K
lLl
mMm
nNn
oOo
pPp
qQq
rRr
sSs
tTt

STARTTEST
:set magic&
:set cpo&
/^TEST/
j:s/A/\='\'/
j:s/B/\='\\'/
j:s/C/\=''/
j:s/D/\='\'/
j:s/E/\='\\'/
j:s/F/\='\r'/
j:s/G/\=' '/
j:s/H/\='\ '/
j:s/I/\='\\ '/
j:s/J/\='\n'/
j:s/K/\="\r"/
j:s/L/\="\n"/
ENDTEST

TEST_3:
aAa
bBb
cCc
dDd
eEe
fFf
gGg
hHh
iIi
jJj
kKk
lLl

STARTTEST
:set magic&
:set cpo&
/^TEST/
j:s/A/\=substitute(submatch(0), '.', '\', '')/
j:s/B/\=substitute(submatch(0), '.', '\\', '')/
j:s/C/\=substitute(submatch(0), '.', '', '')/
j:s/D/\=substitute(submatch(0), '.', '\', '')/
j:s/E/\=substitute(submatch(0), '.', '\\', '')/
j:s/F/\=substitute(submatch(0), '.', '\r', '')/
j:s/G/\=substitute(submatch(0), '.', ' ', '')/
j:s/H/\=substitute(submatch(0), '.', '\ ', '')/
j:s/I/\=substitute(submatch(0), '.', '\\ ', '')/
j:s/J/\=substitute(submatch(0), '.', '\n', '')/
j:s/K/\=substitute(submatch(0), '.', "\r", '')/
j:s/L/\=substitute(submatch(0), '.', "\n", '')/
ENDTEST

TEST_4:
aAa
bBb
cCc
dDd
eEe
fFf
gGg
hHh
iIi
jJj
kKk
lLl

STARTTEST
:set magic&
:set cpo&
/^TEST/
j:s/A\(.\)\(.\)\(.\)\(.\)\(.\)\(.\)\(.\)\(.\)\(.\)/\=submatch(0) . submatch(9) . submatch(8) . submatch(7) . submatch(6) . submatch(5) . submatch(4) . submatch(3) . submatch(2) . submatch(1)/
ENDTEST

TEST_5:
A123456789

STARTTEST
:set magic&
/^TEST/
:set cpo+=/
j:s/A/a/
j:s/B/%/
:set cpo-=/
j:s/C/c/
j:s/D/%/
ENDTEST

TEST_6:
A
B
C
D

STARTTEST
:set magic&
:set cpo&
/^TEST/
j:s/A./\=submatch(0)/
j:s/B./\=submatch(0)/
ENDTEST

TEST_7:
AA
B B

STARTTEST
:g/^STARTTEST/.,/^ENDTEST/d
:1;/^Results/,$wq! test.out
ENDTEST