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

STARTTEST
:so small.vim
ENDTEST

TEST_1:

STARTTEST
:set magic
:set cpo&
:$put =\"\n\nTEST_1:\"
:$put =substitute('A', 'A', '&&', '')
:$put =substitute('B', 'B', '\&', '')
:$put =substitute('C123456789', 'C\(.\)\(.\)\(.\)\(.\)\(.\)\(.\)\(.\)\(.\)\(.\)', '\0\9\8\7\6\5\4\3\2\1', '')
:$put =substitute('D', 'D', 'd', '')
:$put =substitute('E', 'E', '~', '')
:$put =substitute('F', 'F', '\~', '')
:$put =substitute('G', 'G', '\ugg', '')
:$put =substitute('H', 'H', '\Uh\Eh', '')
:$put =substitute('I', 'I', '\lII', '')
:$put =substitute('J', 'J', '\LJ\EJ', '')
:$put =substitute('K', 'K', '\Uk\ek', '')
:$put =substitute('lLl', 'L', '', '')
:$put =substitute('mMm', 'M', '\r', '')
:$put =substitute('nNn', 'N', '\', '')
:$put =substitute('oOo', 'O', '\n', '')
:$put =substitute('pPp', 'P', '\b', '')
:$put =substitute('qQq', 'Q', '\t', '')
:$put =substitute('rRr', 'R', '\\', '')
:$put =substitute('sSs', 'S', '\c', '')
:$put =substitute('uUu', 'U', \"\n\", '')
:$put =substitute('vVv', 'V', \"\b\", '')
:$put =substitute('wWw', 'W', \"\\\", '')
:$put =substitute('xXx', 'X', \"\r\", '')
/^TEST_2
ENDTEST

TEST_2:

STARTTEST
:set nomagic
:set cpo&
:$put =\"\n\nTEST_2:\"
:$put =substitute('A', 'A', '&&', '')
:$put =substitute('B', 'B', '\&', '')
:$put =substitute('C123456789', 'C\(.\)\(.\)\(.\)\(.\)\(.\)\(.\)\(.\)\(.\)\(.\)', '\0\9\8\7\6\5\4\3\2\1', '')
:$put =substitute('D', 'D', 'd', '')
:$put =substitute('E', 'E', '~', '')
:$put =substitute('F', 'F', '\~', '')
:$put =substitute('G', 'G', '\ugg', '')
:$put =substitute('H', 'H', '\Uh\Eh', '')
:$put =substitute('I', 'I', '\lII', '')
:$put =substitute('J', 'J', '\LJ\EJ', '')
:$put =substitute('K', 'K', '\Uk\ek', '')
:$put =substitute('lLl', 'L', '', '')
:$put =substitute('mMm', 'M', '\r', '')
:$put =substitute('nNn', 'N', '\', '')
:$put =substitute('oOo', 'O', '\n', '')
:$put =substitute('pPp', 'P', '\b', '')
:$put =substitute('qQq', 'Q', '\t', '')
:$put =substitute('rRr', 'R', '\\', '')
:$put =substitute('sSs', 'S', '\c', '')
:$put =substitute('tTt', 'T', \"\r\", '')
:$put =substitute('uUu', 'U', \"\n\", '')
:$put =substitute('vVv', 'V', \"\b\", '')
:$put =substitute('wWw', 'W', \"\\\", '')
/^TEST_3
ENDTEST

TEST_3:

STARTTEST
:set magic&
:set cpo&
:$put =\"\n\nTEST_3:\"
:let y = substitute('aAa', 'A', '\="\\"', '')           | $put =y
:let y = substitute('bBb', 'B', '\="\\\\"', '')         | $put =y
:let y = substitute('cCc', 'C', '\=""', '')         | $put =y
:let y = substitute('dDd', 'D', '\="\\"', '')       | $put =y
:let y = substitute('eEe', 'E', '\="\\\\"', '')     | $put =y
:let y = substitute('fFf', 'F', '\="\\r"', '')          | $put =y
:let y = substitute('jJj', 'J', '\="\\n"', '')          | $put =y
:let y = substitute('kKk', 'K', '\="\r"', '')           | $put =y
:let y = substitute('lLl', 'L', '\="\n"', '')           | $put =y
/^TEST_4
ENDTEST

TEST_4:

STARTTEST
:set magic&
:set cpo&
:$put =\"\n\nTEST_4:\"
:let y = substitute('aAa', 'A', '\=substitute(submatch(0), ".", "\\", "")', '')       | $put =y
:let y = substitute('bBb', 'B', '\=substitute(submatch(0), ".", "\\\\", "")', '')     | $put =y
:let y = substitute('cCc', 'C', '\=substitute(submatch(0), ".", "", "")', '')     | $put =y
:let y = substitute('dDd', 'D', '\=substitute(submatch(0), ".", "\\", "")', '')   | $put =y
:let y = substitute('eEe', 'E', '\=substitute(submatch(0), ".", "\\\\", "")', '') | $put =y
:let y = substitute('fFf', 'F', '\=substitute(submatch(0), ".", "\\r", "")', '')      | $put =y
:let y = substitute('jJj', 'J', '\=substitute(submatch(0), ".", "\\n", "")', '')      | $put =y
:let y = substitute('kKk', 'K', '\=substitute(submatch(0), ".", "\r", "")', '')       | $put =y
:let y = substitute('lLl', 'L', '\=substitute(submatch(0), ".", "\n", "")', '')       | $put =y
/^TEST_5
ENDTEST

TEST_5:

STARTTEST
:set magic&
:set cpo&
:$put =\"\n\nTEST_5:\"
:$put =substitute('A123456789', 'A\(.\)\(.\)\(.\)\(.\)\(.\)\(.\)\(.\)\(.\)\(.\)', '\=submatch(0) . submatch(9) . submatch(8) . submatch(7) . submatch(6) . submatch(5) . submatch(4) . submatch(3) . submatch(2) . submatch(1)', '')
/^TEST_6
ENDTEST

TEST_6:

STARTTEST
:set magic&
:$put =\"\n\nTEST_6:\"
:set cpo+=/
:$put =substitute('A', 'A', 'a', '')
:$put =substitute('B', 'B', '%', '')
:set cpo-=/
:$put =substitute('C', 'C', 'c', '')
:$put =substitute('D', 'D', '%', '')
/^TEST_7
ENDTEST

TEST_7:

STARTTEST
:set magic&
:set cpo&
:$put =\"\n\nTEST_7:\"
:$put =substitute('AA', 'A.', '\=submatch(0)', '')
:$put =substitute(\"B\nB\", 'B.', '\=submatch(0)', '')
/^TEST_8
ENDTEST

TEST_8:

STARTTEST
:/^Results/,$wq! test.out
ENDTEST

Results of test72: