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Re: regex alternation

by keszler (Priest)
on Sep 30, 2011 at 10:24 UTC ( #928737=note: print w/replies, xml ) Need Help??

in reply to regex alternation

Why capture the closing single-quote? If you don't need it the alternation is unnecessary overhead. qr<^in directory '/directory/tmp(/\..*)?'$> would be better in that case. Also, if you're not planning to do anything with $1 after the match then there's no need to capture at all: qr<^in directory '/directory/tmp(?:/\..*)?'$>

For overall regex performance it's hard - for me anyway - to determine without knowing what the non-matching data looks like. In general you want the non-matching tests to fail as quickly as possible. (Your anchoring "in directory" to the start of the string is good for that.)

update - moritz's prefix method is even better

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[Corion]: hippo: If you have a Unicode-wise Perl then likely some zero-width characters in $x would work. Maybe $x = "\x{200b}" works.
[Corion]: Hmm - no, that outputs 1 for me on 5.14 - perl -wle "my $x = qq(\x{200b}); warn $x; warn length $x"
[hippo]: Smart - I'll give that a go. Thanks.
[hippo]: Ah
[Corion]: But maybe there is some other Unicode string that will be true but have a zero width
[hippo]: For explanation, I've seen this construct in someone else's code (no names, no pack drill) and couldn't think of a situation to trigger it.
[Corion]: You'll have to look somewhere esoteric for that. Maybe some tied variable or special dualvar can also trigger that. But it's certainly not a common occurrence
[Corion]: And on 5.20, the following also outputs no find:perl -wle 'for my $x ("\x{2000}".."\ x{1fffff}") { if( $x && ! length $x ) { warn qq(<$x>); warn length $x; die } }'
[Corion]: (this time on Unix)
[hippo]: Understood. I'll have to go through the code and see if it's doing anything fancy with ties, dual-vars or non-scalars. In the end, it's probably a bug though.

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