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Re: [GNU m4 1.4.16] Shy groups in regular expressions


From: Tim Landscheidt
Subject: Re: [GNU m4 1.4.16] Shy groups in regular expressions
Date: Mon, 12 Mar 2012 20:49:17 +0000
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/23.1 (gnu/linux)

Eric Blake <address@hidden> wrote:

> [...]

> Shy groups are not part of glibc's re_compile_pattern, and are therefore
> not a part of GNU m4.  M4 is only using glibc's implementation with a
> flags of 0, which happens to be emacs-compatible, and not extensions
> such as shy groups that have later been added to just emacs beyond what
> glibc provides.

> If I could ever get more time to work on m4, there is a proposal for m4
> 2.0 to allow the user to have more control over which flavor of regex is
> in use, rather than forcing things to glibc's re_compile_pattern with
> the default flag settings.  It would be feasible to make m4 2.0 compile
> against libpcre or other such extension mechanism, in order to also
> allow for additional regex flavors with features such as shy groups.

Thanks for the explanation.  While m4 2.0 certainly sounds
nice, I think the bug in:

|      [...]                           The syntax for regular expressions
|      is the same as in GNU Emacs, which is similar to BRE, Basic
|      Regular Expressions in POSIX.  [...]

could also be fixed by changing the reference :-).  I googl-
ed a bit, but gave up when I ended on
<URI:http://www.gnu.org/software/libc/manual/html_mono/libc.html#Regular-Expressions>'s:

| The GNU C library supports two interfaces for matching regu-
| lar expressions.  One is the standard POSIX.2 interface, and
| the other is what the GNU system has had for many years.

and then did not found any documentation for "the other".
Gnulib has some information at
<URI:http://www.gnu.org/software/gnulib/manual/gnulib.html#Regular-expressions>
but it focuses on developers using the C functions rather
than someone interested in the syntax supported.

Tim




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