Re^6: A regex that only matches at offset that are multiples of a given N? (Update:almost perfect!)

that is effectively similar to simply ignoring non-aligned matched ... How so?

Although embedding the called-back code within the regex as a zero-length code assertion looks like its not exiting the regex engine; the effect on performance shows that the need to set-up and tear down a scope and invoke that code has a very significant effect:

$s='a'x1e6; 
cmpthese -1, {
    a => q[ 
        $s =~ m[(.)(?{$c=pos();})]g;
    ],
    b => q[ 
        $s =~ m[(.)]g;
    ], 
};;
       Rate       a       b
a    3924/s      --   -100%
b 4206322/s 107088%      --
[download]

If it was possible to avoid running 3 orders of magnitude slower, the benefits would be enormous for my application.

With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.

"Science is about questioning the status quo. Questioning authority".

In the absence of evidence, opinion is indistinguishable from prejudice.

Comment on Re^6: A regex that only matches at offset that are multiples of a given N? (Update:almost perfect!) Download Code

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Re^7: A regex that only matches at offset that are multiples of a given N? (Update:almost perfect!) by smls (Friar) on Feb 13, 2013 at 21:11 UTC
If it was possible to avoid running 3 orders of magnitude slower Well, in this benchmark the embedded code gets called on every single character in the string, while in the problem at hand it would only get called once for every correct match. Finding the match in the first place would happen completely within the regex engine. If you are using the regex with a while loop, you could move the pos()-incrementing into the already existing loop body to avoid creating an additional scope: `while( $input =~ m{\G(?:.{$n})?(?=fred(....))}g ) { pos($input) += $n; # ... # do stuff with $1 # ... }` [download] This could also be adapted to ensure that new occurrences of "fred" may not be matched inside the `(....)` that belongs to the previous match, by modifying the pos()-incrementing line like this: `pos($input) += $n int((length('fred'.$1) - 1) / $n + 1);` [download]	[reply] [d/l] [select]
Re^8: A regex that only matches at offset that are multiples of a given N? (Update:almost perfect!) by BrowserUk (Patriarch) on Feb 13, 2013 at 23:12 UTC
You're right! Moving the pos modification into the while loop body means that it only gets executed after successful (aligned) matches. And that helps my performance a lot. Thank you. Especially for persisting when I wasn't seeing the wood for the trees. With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday' Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error. "Science is about questioning the status quo. Questioning authority". In the absence of evidence, opinion is indistinguishable from prejudice.	[reply]