Test for *sub-replace-special* and *sub-replace-expression* on :substitute.
Test for submatch() on :substitue.
Test for *:s%* on :substitute.
Test for :s replacing \n with  line break.

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/ /
j:s/U/\L\uuUu\l\EU/
j:s/V/\U\lVvV\u\Ev/
ENDTEST

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

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/ /
j:s/U/\L\uuUu\l\EU/
j:s/V/\U\lVvV\u\Ev/
ENDTEST

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

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)/
j:s/B\(.\)\(.\)\(.\)\(.\)\(.\)\(.\)\(.\)\(.\)\(.\)/\=string([submatch(0, 1), submatch(9, 1), submatch(8, 1), submatch(7, 1), submatch(6, 1), submatch(5, 1), submatch(4, 1), submatch(3, 1), submatch(2, 1), submatch(1, 1)])/
ENDTEST

TEST_5:
A123456789
B123456789

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_7/
j:s/A./\=submatch(0)/
j:s/B./\=submatch(0)/
j:s/C./\=strtrans(string(submatch(0, 1)))/
j:s/D.\nD/\=strtrans(string(submatch(0, 1)))/
j:s/E\_.\{-}E/\=strtrans(string(submatch(0, 1)))/
/^Q$
:s/Q[^\n]Q/\=submatch(0)."foobar"/
:" Avoid :s error breaks dotest map on Windows.
ENDTEST

TEST_7:
AA
B B
C C
D 
D
E 
 
 
 
 E
Q
Q

STARTTEST
:function! TitleString()
  let check = 'foo' =~ 'bar'
  return ""
endfunction
:set titlestring=%{TitleString()}
:/^test_one/s/.*/\="foo\nbar"/
:/^test_two/s/.*/\="foo\nbar"/c
y
ENDTEST

test_one
test_two

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