Stripping whitespace from both ends of a string…

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I was just in a room with three competent, professional Perl developers, all of whom agreed that you can’t strip whitespace from both ends of a string in a single regexp.

The common knowledge is:

$str =~ s/^\s+//;

$str =~ s/\s+$//;

I fiddled around and found this:

$str =~ s/^\s*(.*?)\s*$/$1/;

I think it’s right.  Is it better?  Not sure.   Is it faster?  Slower?  Clearer?

Speed I can test with benchmark… and using two is faster.

% perl
Rate single multi
single 444444/s — -74%
multi 1724138/s 288% —

So, nevermind.  Use two regexps.

Is one clearer?  Not sure.  But it’s slower!


13 Responses to “Stripping whitespace from both ends of a string…”

  1. chackoc says:

    $str =~ s/(^\s+|\s+$)//g;

  2. brunov says:

    For the epitome of clear:

    use perl5i::latest;


  3. Adam Kennedy says:

    $str =~ s/(?:^\s+|\s+$)//g;

  4. zloyrusskiy says:

    I’m using:


    it’s one regular expression + has good speed

  5. Mark Grimes says:

    What about:

    $str =~ s/^\s*|\s*$//g;

    I can’t test or benchmark it right now, but I know I’ve used something along these lines before.

  6. Yanick says:

    What about

    $str =~ s/^\s+|\s+$//g


  7. Yanick says:

    What about

    $str =~ s/^\s+|\s+$//g


    It’s faster, and not too obtuse.

  8. Dave Rolsky says:

    Yes, it’s slower, but look at the absolute speed. The “slow” version can still execute 440,000 or so per minute!

    Write code as clearly as possible, _then_ optimize the slow parts. This is what profilers are for.

    This sort of premature micro-optimization makes for messy code that may actually be no faster than clean code.

    Also, I think the simplest way to write it is …

    $str =~ s/^\s+|\s+$//g;

  9. Hercynium says:

    What about:

    $str =~ s/^\s*|\s*$//g

    Seems clearer and will probably be faster.

  10. mattp says:

    its slower due to the backtracking required from the combination of two greedy quantifiers with the one non greedy.
    to quote knuth .. premature optimization is the root of all evil

  11. dami says:

    Jeffrey Friedl recommends using 2 regexes. But you can do it in one line :

    s/^\s+//, s/\s+$// for $str;

  12. John Wiersba says:

    I think the hands down winner is $str =~ s/^\s+//; $str =~ s/\s+\z//;

    It says exactly what you want and it’s the fastest. No need to “optimize” with a single regex. Although dami’s postfix-for form is 7 characters shorter, the need for a comma instead of semicolon is a blemish, plus it’s about 25% slower.

    #!/usr/bin/perl -w
    use strict;
    use Benchmark;
    use vars qw($str1);
    $str1 = ” asf asd fasd fasd f asdf asd asdf as fsda fasdf “;
    timethese( -5, {
    test1_0 => ‘ $str = $str1; $str =~ s/^\s+|\s+\z//g ‘,
    test1_1 => ‘ $str = $str1; $str =~ s/^\s*|\s*\z//g ‘,
    test1_2 => ‘ $str = $str1; $str =~ s/^\s+|\s+$//g ‘,
    test1_3 => ‘ $str = $str1; $str =~ s/^\s*|\s*$//g ‘,
    test2_0 => ‘ $str = $str1; $str =~ s/^\s+//; $str =~ s/\s+\z// ‘,
    test2_1 => ‘ $str = $str1; $str =~ s/^\s+//; $str =~ s/\s+$// ‘,
    test2_2 => ‘ $str = $str1; $str =~ s/^\s*//; $str =~ s/\s*\z// ‘,
    test2_3 => ‘ $str = $str1; $str =~ s/^\s*//; $str =~ s/\s*$// ‘,
    test3 => ‘ $str = $str1; s/^\s+//, s/\s+\z// for $str ‘,

    Benchmark: running test1_0, test1_1, test1_2, test1_3, test2_0, test2_1, test2_2, test2_3, test3 for at least 5 CPU seconds…
    test1_0: 6 wallclock secs ( 5.29 usr + 0.00 sys = 5.29 CPU) @ 185981.10/s (n=983840)
    test1_1: 5 wallclock secs ( 5.22 usr + 0.00 sys = 5.22 CPU) @ 154480.84/s (n=806390)
    test1_2: 6 wallclock secs ( 5.22 usr + 0.00 sys = 5.22 CPU) @ 188475.10/s (n=983840)
    test1_3: 5 wallclock secs ( 5.25 usr + 0.00 sys = 5.25 CPU) @ 154564.00/s (n=811461)
    test2_0: 5 wallclock secs ( 5.26 usr + 0.01 sys = 5.27 CPU) @ 672072.87/s (n=3541824)
    test2_1: 6 wallclock secs ( 5.44 usr + 0.00 sys = 5.44 CPU) @ 651070.77/s (n=3541825)
    test2_2: 5 wallclock secs ( 5.17 usr + 0.00 sys = 5.17 CPU) @ 248851.26/s (n=1286561)
    test2_3: 5 wallclock secs ( 5.23 usr + 0.00 sys = 5.23 CPU) @ 241523.71/s (n=1263169)
    test3: 4 wallclock secs ( 5.29 usr + 0.00 sys = 5.29 CPU) @ 526948.02/s (n=2787555)

  13. […] noticed a couple blog entries in the Planet Perl Iron Man feed discussing which way of stripping whitespace from […]

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