bobf has asked for the wisdom of the Perl Monks concerning the following question:

I have a string that begins with a variable sequence of characters, which is followed by a constant string that can be used as a unique anchor. I want to strip off the variable stuff at the beginning so that it begins with the anchor. For example, given this: 

my $s = 'variable chars anchor want this';

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I want to end with this: 
$s = 'anchor want this';

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I can think of several ways to approach this, but I do not know which might be considered better than others from the standpoint of style or efficiency (not just benchmarking, but also including potential for backtracking). These approaches include:

All of these employ .* (except for the silly constructs, which are left as an exercise for the reader), but I am trying to avoid that idiom by thinking more carefully about my regexen (Death to Dot Star!). I suspect a better regex might be something like a negated character class, but for a group: "match everything from the start of the string that does not match the sequence of characters anchor".

How would you do this, and why?

Replies are listed 'Best First'.
Re: Regex style and efficiency
by BrowserUk (Patriarch) on Jan 10, 2010 at 22:13 UTC

    Because it is simple and efficient: 

    $s = 'variable chars anchor want this';;
$s = substr $s, index $s,'anchor';;
print $s;;
anchor want this

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    If I was going to use a regex (say the anchor had a variable component), then: 

    $s = 'variable chars anchor want this';;
$s =~ s[.+(?=anchor)][];;
print $s;;
anchor want this

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    The difference: 

    cmpthese -1,{
    a=> q[$s='variable chars anchor want this';$s=~s[.+(?=anchor)][];]
+,
    b=> q[$s='variable chars anchor want this';$s=substr $s, index $s,
+'anchor';]
};;
       Rate    a    b
a 1592446/s   -- -48%
b 3055291/s  92%   --

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      As is, the substr/index version assumes the anchor is present. That may or may not be a problem.
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      Nice. It didn't even occur to me to use substr/index for this. I also like how you used the non-capturing look-ahead to prevent the desired text from being included in the s///. Thanks!

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Re: Regex style and efficiency
by JavaFan (Canon) on Jan 11, 2010 at 10:02 UTC
    Here's one that doesn't use .*: 
    $s =~ /anchor/p and $s = ${^MATCH} . ${^POSTMATCH};

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