Hi,
the documentation for regexp says:
| -- 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. *Note Syntax of Regular
|
Eric Blake 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 j
