Jan Engelhardt <[email protected]> wrote:
> On Aug 16 2007 10:21, Andy Whitcroft wrote:
>>> + if ($line =~ /\bif\s*\([^\)]*\)\s*\;/) {
>>
>>Heh, you are the second person to suggest this check today, do I detect
>>some ripped out hair due to one of these!
>>
>>I've taken this idea and expanded it to cover if, for and while which
>>can all suffer from this. Using the relative indent to work out which
>>are valid combinations:
>
> 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).
--
Your e-mail has been returned due to insufficient voltage.
Fri?, Spammer: [email protected] [email protected]
[email protected] [email protected]
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(??{ $r })?b/;
if ($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
>--
>Your e-mail has been returned due to insufficient voltage.
>
>Friß, Spammer: [email protected] [email protected]
> [email protected] [email protected]
>-
>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/
>
Jan
--
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
On Aug 22 2007 11:21, 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.
Exactly, and which is why my idea was to use a (??{ }) block to match if((()));
but for some reason, it did not fly, and I do not know either why.
Jan
--