(Ovid - Common regex error) RE: A two-liner for Backtracking for substitutions

I didn't go over your code in detail, but I did notice a common regex error:

[\w\d]
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Many people (including yours truly at one time), mistakenly assume that \w does not match 0-9. Surprise! It does. This caused me a problem when I was trying to do the following:

my $text = "product1234imageSmall.jpg";
($type, $id, $property) = ($1, $2, $3) if $text =~ /^(\w+)(\d+)(\w+)/;
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It failed pretty quickly because $type was getting set to product123 (it didn't pick up the "4" because \d had to match something).

In this case, because you are including both \w and \d in a character class, there's only an issue of redundancy and doesn't affect the functioning of the regex. I just wanted to point this out because it's easy to miss that and get bitten in other situations.

Cheers,
Ovid

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RE: (Ovid - Common regex error) RE: A two-liner for Backtracking for substitutions by markwild (Sexton) on Oct 03, 2000 at 02:44 UTC
Thanks Ovid! I appreciate the friendly amendment. How does the rest of the code look? I'd also like to hear from the original anonymous poster. Did he get his problem solved? --Mark	[reply]
(Ovid - Regex efficiency issues) RE(3): A two-liner for Backtracking for substitutions by Ovid (Cardinal) on Oct 03, 2000 at 03:10 UTC
There are a few problems with your code. However, if I write a book, I'm going to dedicate it to: `#!/usr/bin/perl -w use strict;` [download] Those two little lines have saved me more trouble than you can possibly imagine and I would strongly recommend that you incorporate them. Admittedly, you just posted what you did for testing purposes, but I still have this "knee jerk" reaction regarding anything without the `-w` switch or `use strict`. Your first regex can be made a bit more efficient (and accurate) by eliminating the ., matching to the beginning of the line and using the /m switch: `$mydata =~ s/^([\w\s]+)\s([\w]+)\s(0000)/$1,$2,$3/mg;` [download] I haven't actually benchmarked this, but I'd bet good money that this is the case. See Death to Dot Star! for information on why . is problematic. The accuracy issue is probably a mute point if you have relatively clean data. The second regex has two issues. You forgot to put parentheses around the `\s0000`. Those parentheses were supposed to capture this data and substitute it back using $2. I just changed it to the following: `$mydata =~ s/(:\d\d)\s0000/$1, 0000/g;` [download] The other problem is a really just a minor efficiency issue: `\d{2}` is better written as `\d\d` (this is from MRE, so it may be out of date for newer regex engines). Basically, when you use `\d{2}`, the regex engine is forced to keep track of the number of instances of `\d`. This slows it down just a tad (which can be significant when iterating over a large amount of data). However, when the regex engine sees `\d\d`, it just matches each instance of `\d` which is faster. Hope this helps! Cheers, Ovid Join the Perlmonks Setiathome Group or just go the the link and check out our stats.	[reply] [d/l] [select]