[GNU m4 1.4.16] Shy groups in regular expressions

[Tim Landscheidt]

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
|      Expressions: (emacs)Regexps.  Support for ERE, Extended Regular
|      Expressions is not available, but will be added in GNU M4 2.0.

|      [...]

However:

| address@hidden ~]$ m4
| regexp(`abc', `\(b\)')
| 1
| regexp(`abc', `\(?:b\)')
| -1
| address@hidden ~]$

Emacs's documentation on "Backslash in Regular Expressions"
that is linked from m4's info file doesn't seem to imply
that shy groups were in fact ERE.  So is this a bug?

Tim