On Mon, 20 Aug 2007, Jan Engelhardt wrote: > On Aug 20 2007 13:52, Bodo Eggert wrote:
> >> But. The above regex does not seem to handle > >> > >> if ((a = b)); > >> oops; > >> > >> I have tried to come up with a superduper regex that handles multiple > >> (), but my regex fu seems to stop above two pairs of (). > > > >This is because you can't do that using finite regular expressions. > > > >Regular expressions are Type-3 grammars, but you'd need a Type-2 > >grammar to express the Dyck language (and you need to parse a Dyck > >Language, ignoring the non-dyck-parts). > > So what about this then... > > > $s = shift @ARGV; > $r = qr/a(??{ $r })?b/; This is not a regular expression, because it can't be parsed by a finite state machine (DFA/NFA) without a stack. http://en.wikipedia.org/wiki/Deterministic_finite_state_machine Obviously perl does allow non-regular expressions. > if ($s =~ /^$r$/) { > print "Yup, that's good\n"; > } else { > print "fail\n"; > } > > > $ perl foo.pl aabbbb > Not so much > $ perl foo.pl aaaabbbb > Yup, that's good > $ perl foo.pl aaaaabbbb > Not so much perl foo.pl aaababbb fail "$r = qr/a(??{ $r })?b(??{ $r })?/;" does seem to work. -- "Those who would give up essential liberty, to purchase a little temporary safety, deserve neither liberty nor safety." -- Benjamin Franklin, Historical Review of Pennsylvania, 1759 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/