regex capture group names must use identifier syntax

[khwilliamson]

Prior to this the non-first characters in a capture group name could be any \w character, though they were supposed to follow perl identifier syntax. But an identifier excludes a few \w characters from appearing in them. This p.r. tightens what is allowed.

#23775 gave a list of them, but I forgot a couple details in generating that list, so it wasn't quite right.

The complete corrected list is:

GREEK YPOGEGRAMMENI COMBINING CYRILLIC HUNDRED THOUSANDS SIGN COMBINING CYRILLIC MILLIONS SIGN COMBINING PARENTHESES OVERLAY COMBINING ENCLOSING CIRCLE COMBINING ENCLOSING SQUARE COMBINING ENCLOSING DIAMOND COMBINING ENCLOSING CIRCLE BACKSLASH COMBINING ENCLOSING SCREEN COMBINING ENCLOSING KEYCAP COMBINING ENCLOSING UPWARD POINTING TRIANGLE CIRCLED LATIN CAPITAL LETTER A - Z CIRCLED LATIN SMALL LETTER A - Z VERTICAL TILDE COMBINING CYRILLIC TEN MILLIONS SIGN COMBINING CYRILLIC HUNDRED MILLIONS SIGN COMBINING CYRILLIC THOUSAND MILLIONS SIGN ARABIC LIGATURE SHADDA WITH DAMMATAN ISOLATED FORM ARABIC LIGATURE SHADDA WITH KASRATAN ISOLATED FORM ARABIC LIGATURE SHADDA WITH FATHA ISOLATED FORM ARABIC LIGATURE SHADDA WITH DAMMA ISOLATED FORM ARABIC LIGATURE SHADDA WITH KASRA ISOLATED FORM ARABIC LIGATURE SHADDA WITH SUPERSCRIPT ALEF ISOLATED FORM ARABIC LIGATURE SALLALLAHOU ALAYHE WASALLAM ARABIC LIGATURE JALLAJALALOUHOU ARABIC FATHATAN ISOLATED FORM ARABIC DAMMATAN ISOLATED FORM ARABIC KASRATAN ISOLATED FORM ARABIC FATHA ISOLATED FORM ARABIC DAMMA ISOLATED FORM ARABIC KASRA ISOLATED FORM ARABIC SHADDA ISOLATED FORM ARABIC SUKUN ISOLATED FORM SQUARED LATIN CAPITAL LETTER A - Z NEGATIVE CIRCLED LATIN CAPITAL LETTER A - Z NEGATIVE SQUARED LATIN CAPITAL LETTER A - Z

• This set of changes requires a perldelta entry