Hi, (This may be relevant to the m4 list as well, which I'm not currently subscribed to.) tl;dr: ====== I think that the autoconf and m4 manuals could have improved cross-references to the regular expression syntax actually supported in autoconf via GNU m4: autoconf: - m4_bpatsubst(): - Cross-reference to M4's patsubst(). - Possibly cross-reference to Gnulib's emacs-style regex syntax. - m4_bregexp(): - Cross-reference to M4's regexp(). - Possibly cross-reference to Gnulib's emacs-style regex syntax. m4: - regexp: - Cross-reference to Gnulib's emacs-style regex syntax. - patsubst: - Already internally references to m4's regexp, so covered above? Thoughts? Details: ======== While debugging an issue with m4_bregexp() (and m4_bpatsubst()), I ended up in a twisty maze of documentation trying to understand the specific regex features available in autoconf and GNU m4. 1. autoconf section 8.3.1 'Redefined M4 Macros' https://www.gnu.org/savannah-checkouts/gnu/autoconf/manual/autoconf-2.71/html_node/Redefined-M4-Macros.html Macro: m4_bregexp (string, regexp, [replacement]) This macro corresponds to regexp. The name m4_regexp is kept for future versions of M4sugar, once GNU M4 2.0 is released and supports extended regular expression syntax. There's no specific cross-link here to the m4 macros. 2. m4 section 11.3 'Searching for regular expressions' https://www.gnu.org/software/m4/manual/html_node/Regexp.html Builtin: regexp (string, regexp, [replacement]) Searches for regexp in string. The syntax for regular expressions is the same as in GNU Emacs, which is similar to BRE, Basic Regular Expressions in POSIX. See [Syntax of Regular Expressions] in the GNU Emacs Manual. Support for ERE, Extended Regular Expressions is not available, but will be added in GNU M4 2.0. [Syntax of Regular Expressions] is a link to https://www.gnu.org/software/emacs/manual/emacs.html#Regexps which goes to the top of the Emacs manual. After a bit of poking around I found Emacs manual section 15.6 - see below Code inspection of GNU m4 reveals that m4 uses Gnulib's regex implementation in the default (Emacs-like) regex mode; src/builtin.c (etc) don't call re_set_syntax() and the default appears to be RE_SYNTAX_EMACS 0. See item 5 below. 3. Emacs section 15.6 'Syntax of Regular Expressions' https://www.gnu.org/software/emacs/manual/html_node/emacs/Regexps.html This section (and this manual in general) describes regular expression features that users typically use. See [Regular Expressions] in The Emacs Lisp Reference Manual, for additional features used mainly in Lisp programs. [Regular Expressions] is a link to https://www.gnu.org/software/emacs/manual/html_node/elisp/Regular-Expressions.html#Regular-Expressions 4. Emacs section 35.3 'Regular Expressions' https://www.gnu.org/software/emacs/manual/html_node/elisp/Regular-Expressions.html contains section 35.3.1 'Syntax of Regular Expressions' https://www.gnu.org/software/emacs/manual/html_node/elisp/Syntax-of-Regexps.html contains section 35.3.1.3 'Backslash Constructs in Regular Expressions' https://www.gnu.org/software/emacs/manual/html_node/elisp/Regexp-Backslash.html which describes various backslash features. As far as I can tell, some of these aren't implemented by GNU m4: - \{ and \} for intervals. - \(?: ... \) for shy groups. - \(?num ... \) for explicitly numbered groups. 5. Gnulib section 18.7.4 '‘emacs’ regular expression syntax' https://www.gnu.org/software/gnulib/manual/html_node/emacs-regular-expression-syntax.html This describes various regex features in Gnulib's emacs emulation. Supports \( \) grouping, \| alternation, some GNU extensions, but not \{ \} intervals from Emacs / POSIX BREs. Regards, Luke.
