Re^3: Regex code block executes twice per match using look-arounds

test look-behind, '}' so succeeds test look-ahead, '[' also succeeds code block encountered, execute advance pointer one place
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That last step is wrong. It should be

advance pointer the length of the match
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The length of the match is zero in the case where the lookahead and lookbehind is used. Since the pointer is not advanced, the regexp matches everything twice. Only a final check prevents the regexp from returning an identical match.

test look-behind, '}' so succeeds
test look-ahead, '[' also succeeds
code block encountered, execute
same match? no, continue
advance pointer the length of the match (0)

test look-behind, '}' so succeeds
test look-ahead, '[' also succeeds
code block encountered, execute
same match? yes, backtrack
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Re^4: Regex code block executes twice per match using look-arounds by johngg (Canon) on Jul 12, 2007 at 19:20 UTC
`advance pointer the length of the match` That was the fundamental piece of the puzzle that I was missing. Indeed, after jettero's 2nd reply I started to investigate with `use re 'debug';` and could see the mechanism you describe. I changed the regex so that it was using the look-behind but the look-ahead was replaced with a simple capture `... my $rxBetween = qr {(?x) (?<=($rxClose)) ($rxOpen) (?{print qq{Match @{ [++ $count] }: on left $1, on right $2\n}} +) }; ... $string =~ s{$rxBetween}{+$2}g; ...` [download] and that stopped the double execution. It also seemed clear from the debug output that using both look-arounds was making the engine do a lot more work. Read more... (11 kB) Thank you for your replies and the insights they have given. Cheers, JohnGG	[reply] [d/l] [select]

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Re^4: Regex code block executes twice per match using look-arounds
by johngg (Canon) on Jul 12, 2007 at 19:20 UTC

advance pointer the length of the match

That was the fundamental piece of the puzzle that I was missing. Indeed, after jettero's 2nd reply I started to investigate with use re 'debug'; and could see the mechanism you describe.

I changed the regex so that it was using the look-behind but the look-ahead was replaced with a simple capture

...
my $rxBetween = qr
   {(?x)
       (?<=($rxClose))
       ($rxOpen)
       (?{print qq{Match @{ [++ $count] }: on left $1, on right $2\n}}
+)
   };
...
$string =~ s{$rxBetween}{+$2}g;
...
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and that stopped the double execution. It also seemed clear from the debug output that using both look-arounds was making the engine do a lot more work.