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Answer: How do I remove whitespace at the beginning or end of my string?
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Q&A > regular expressions > How do I remove whitespace at the beginning or end of my string? contributed by Roy Johnson

I wrote some benchmark code to test the popular answers, and a method using just pattern match and capture instead of s///. I got good results from my pattern match, so I tried plugging my pattern into Daddio's solution (s-capture), and got a significant speedup of it (s-capture2), but still not as good as just using m//. 
use Benchmark 'cmpthese';
use Regexp::Common 'whitespace';

my $f = '   this is a string with  spaces  to    remove      ';
cmpthese(-3, {
  'Regexp-Common' => sub { $_=$f; s/$RE{ws}{crop}//g;    },
  'two-s///'      => sub { $_=$f; s/^\s+//; s/\s+$//;    },
  'one-s///'      => sub { $_=$f; s/^\s+|\s+$//g;        },
  's-capture'     => sub { $_=$f; s/^\s*(.*?)\s*$/$1/;   },
  's-capture2'    => sub { $_=$f; s/^\s*(\S+(?:\s+\S+)?)?\s*$/$1/; },
  'm-capture'     => sub { $_=$f; ($_) = /(\S+(?:\s+\S+)?)/;    },
});

[download]
                 Rate Regexp-Common s-capture one-s/// s-capture2 m-ca
+pture two-s///
Regexp-Common   659/s            --      -91%     -93%       -95%     
+ -96%     -97%
s-capture      7002/s          963%        --     -21%       -45%     
+ -59%     -73%
one-s///       8857/s         1244%       26%       --       -30%     
+ -48%     -66%
s-capture2    12699/s         1827%       81%      43%         --     
+ -26%     -51%
m-capture     17179/s         2507%      145%      94%        35%     
+   --     -34%
two-s///      25941/s         3837%      270%     193%       104%     
+  51%       --

[download]
Not surprisingly, the straightforward method of calling s/// twice is the fastest. What is surprising is how slow Regexp-Common turns out to be, and I was pleasantly surprised at how well m-capture did. If, for some bizarre reason, you needed a single atomic expression to remove leading and trailing whitespace, that would seem to be the way to go.

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Re: Answer: How do I remove whitespace at the beginning or end of my string?
by ysth (Canon) on Jan 27, 2004 at 22:17 UTC
    Why are you penalizing the one-regex methods by requiring a separate assignment to $_? You should perhaps do 2 separate benchmarks; one assuming $_ is to be changed, and the other assuming a lexical $f is to be changed.

    Update: I'm just saying that benchmarks for performing an operation on $_ won't necessarily carry over to benchmarks for performing an operation on a lexical. In fact, with a lexical, the two-regex method outshines the others even more: 

    use Benchmark 'cmpthese';
use Regexp::Common 'whitespace';

my $f = '   this is a string with  spaces  to    remove      ';
cmpthese(-3, {
  'Regexp-Common' => sub { my $g=$f; $g=~s/$RE{ws}{crop}//g;    },
  'two-s///'      => sub { my $g=$f; $g=~s/^\s+//; $g=~s/\s+$//;    },
  'one-s///'      => sub { my $g=$f; $g=~s/^\s+|\s+$//g;        },
  's-capture'     => sub { my $g=$f; $g=~s/^\s*(.*?)\s*$/$1/;   },
  's-capture2'    => sub { my $g=$f; $g=~s/^\s*(\S+(?:\s+\S+)?)?\s*$/$
+1/; },
  'm-capture'     => sub { my $g=$f; ($g) = $g=~/(\S+(?:\s+\S+)?)/;   
+ },
});

    [download]
    shows "two-s" gaining more than the others when working on lexicals.
[reply]
[d/l]
      The test is to remove leading and trailing whitespace from a string. The assignment to $_ is an initialization condition of the test: it has to have the string value in it, and then have the whitespace removed.

      Update:
      Oh, now I understand what you're saying. It just didn't occur to me that it could make a significant difference what kind of variable was being operated on.

      The PerlMonk tr/// Advocate
[reply]
Correction: How do I remove whitespace at the beginning or end of my string?
by Roy Johnson (Monsignor) on Jan 28, 2004 at 18:39 UTC
    Argh! The m-capture regex has a bug: the final ? should be a * (and the corresponding ? in s-capture2). Naturally, this slows things down a bit (and I can't update the original answer).
    Corrected code: 
      's-capture2'    => sub { $_=$f; s/^\s*(\S+(?:\s+\S+)*)?\s*$/$1/; },
  'm-capture'     => sub { $_=$f; ($_) = /(\S+(?:\s+\S+)*)/;    },

    [download]
    New results (on a different machine): 
                      Rate Regexp-Common s-capture one-s/// s-capture2 m-c
+apture two-s///
Regexp-Common   6629/s            --      -89%     -90%       -91%    
+  -94%     -97%
s-capture      62950/s          850%        --      -1%       -18%    
+  -41%     -71%
one-s///       63621/s          860%        1%       --       -17%    
+  -41%     -71%
s-capture2     77016/s         1062%       22%      21%         --    
+  -28%     -65%
m-capture     107187/s         1517%       70%      68%        39%    
+    --     -51%
two-s///      219320/s         3209%      248%     245%       185%    
+  105%       --

    [download]
    The PerlMonk tr/// Advocate
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