linux-kernel - Re: [PATCH] Make checkpatch rant about trailing ; at the end of "if" expr

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Date:	Wed, 22 Aug 2007 11:21:48 +0200 (CEST)
From:	Bodo Eggert <7eggert@....de>
To:	Jan Engelhardt <jengelh@...putergmbh.de>
cc:	Bodo Eggert <7eggert@....de>, Andy Whitcroft <apw@...dowen.org>,
	Eugene Teo <eugeneteo@...nel.sg>, linux-kernel@...r.kernel.org,
	auke-jan.h.kok@...el.com
Subject: Re: [PATCH] Make checkpatch rant about trailing ; at the end of "if"
  expr

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
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