I should have qualified my statement by saying that standard regular grammars cannot handle this sort of pattern, whereas Perl's regexes can do everything and anything.

In fact embedded actions within a Perl regex can do anything Perl can do - Therefore Perl regex's can do anything Perl can do. :)

I guess I can see from your use of Parse::RecDescent, Update: And from re-reading the very long manual last night, that the answer to my question is something like:

my $grammar = q{
   match : part(s) { print join('', @{$item[1]}) }

   part : AnB
   part : 'a'
   part : /[^a]+/

   AnB : 'a' AnB 'b' { 'c' . $item[2] . 'd' }
   AnB : 'ab' { 'cd' }
}
[download]

-Andrew.