regexps

[Akim Demaille]

I hate BREs.  I know Marc had to implement some -G in OpenBSD M4 to
support the broken BREs from GNU M4.

Would it be possible to find some means to have the two M4 compatible?
AFA Autoconf is concerned, I certainly would like ERE better.

Of course the main issue is compatibility.



GNU M4 backward compatibility can be handled by several means: new
options, and/or new patsubst/regex macros (with new names), and/or
changesyntax extensions.

New options is not enough for Autoconf: its core will sure move
quickly to ere, as bre are considerably obfuscating the code, but user
code will take more time.  So in the case of Autoconf I know exactly
what I will do: have m4_regex and m4_patsubst be ERE (since I want the
users to drop non m4_* macros dead), and patsubst & regex be BRE +
warning.

This will be easy whatever set of macros is provided (changesyntax,
changeregex, or addition of new ere macros).



But the OpenBSD vs GNU compatibility with this regard is definitely in
pain, as -G means completely different things in each world...