Re^3: Explain a regexp matched group result

$-[0] is the offset of the start of the last successful match.

You have my $re1 = qr/((a+)?(b+)?(c))*/;

Your outer capture group may be repeated zero or more times. In the case of your test string, "aacbbbcac", it matches three times:

At the first repeat, it matches "aac" with $1 being "aac", $2 being "aa", $3 being undef (not matched) and $4 being "c", but because of the repeat count it doesn't stop there, so you never see these values.

At the second repeat $1 is "bbbc", $2 remains "aa" (group (a+) didn't match in this repeat but $2 is the 'last successful match'), $3 is "bbb" and $4 is "c", but it doesn't stop there, so you don't see these values either.

The third and final repeat sets $1 to "ac", $2 to "a" leaves $3 as it was at the last successful match (i.e. "bbb") and sets $4 to "c".

So, the issue is that the capture groups return the last successful match rather than the last match or failure as the case may be.

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Re^4: Explain a regexp matched group result by jdd (Acolyte) on Oct 28, 2013 at 19:24 UTC
Thank you very much. So I cannot get individuals without removing the repetition and do a while ()... i.e. `use strict; use warnings FATAL => 'all'; my $string = "aacbbbcac"; my $re1 = qr/((a+)?(b+)?(c))/; while ($string =~ /$re1/g) { foreach (0..$#-) { printf "Group %d: <%s>\n", $_, defined($-[$_]) ? substr($string, $ +-[$_], $+[$_] - $-[$_]) : ''; } print "\n"; }` [download] Bad news for me, but again mega thanks for your very clear answer. ps: `Group 0: <aac> Group 1: <aac> Group 2: <aa> Group 3: <> Group 4: <c> Group 0: <bbbc> Group 1: <bbbc> Group 2: <> Group 3: <bbb> Group 4: <c> Group 0: <ac> Group 1: <ac> Group 2: <a> Group 3: <> Group 4: <c>` [download]	[reply] [d/l] [select]
Re^5: Explain a regexp matched group result by ig (Vicar) on Oct 28, 2013 at 19:31 UTC
I think your conclusion is correct in general, but if you know the structure of the RE then there are workarounds to the way the capture groups work. Consider: #!C:/strawberry/perl/bin/perl.exe # use strict; use warnings; my $string = "aacbbbcac"; my $re1 = qr/((a+)?(b+)?(c))/; #my $re1 = qr/((a)(b)(c))/; #my $re1 = qr/((a+)?(b)(c))/; if ($string =~ $re1) { my $start = 0; my @something; foreach (0..$#-) { if(defined($-[$_])) { $start = $-[$_] if($-[$_] > $start); if($-[$_] >= $start) { printf "Group %d: <%s>\n", $_, substr($string, $-[$_], $+[ +$_] - $-[$_]); $something[$_] = substr($string, $-[$_], $+[$_] - $-[$_]); } else { printf "Group %d: <%s> - but ignore it because it is from +a previous iteration of the outer capture group\n", $_, substr($strin +g, $-[$_], $+[$_] - $-[$_]); $something[$_] = ''; } } else { printf "Group %d: hasn't matched yet\n", $_; $something[$_] = ''; } } print "$1 = " . join('', @something[2..4]) . "\n"; } [download] Which produces `Group 0: <aacbbbcac> Group 1: <ac> Group 2: <a> Group 3: <bbb> - but ignore it because it is from a previous iteration + of the outer capture group Group 4: <c> ac = ac` [download] If you are trying to write something that handles arbitrary REs, this approach is unlikely to work.	[reply] [d/l] [select]
Re^6: Explain a regexp matched group result by jdd (Acolyte) on Oct 28, 2013 at 19:45 UTC
Formidable! How to say yes: I want to handle arbitraries REs but only REs that passed my grammar, so I know in advance the overall structure. Complicated but perfectly doable. The origin of my question is the comparison of perl's regexp with ECMAScript regexp, namely Note 3 of chapter 15.10.2.5 of ECMA-262 grammar spec (a perfectlty sensible question since ECMA regexp are are copy of perl5's). I have done the AST of any ECMAScript source (c.f. MarpaX::Languages::ECMA::AST) so now I was wondering about the actions associated to the grammar.	[reply]
Re^7: Explain a regexp matched group result by ig (Vicar) on Oct 28, 2013 at 20:52 UTC


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