IMHO, this is actually bad advice. /o can cause some confusing bugs and is generally a case of premature optimization. If I run the benchmark code: 
#!/usr/bin/perl
use strict;
use warnings;

use Benchmark 'cmpthese';

local $" = '|';
my $target      = join '', map chr(97 + rand 26), 1 .. 100000;
my @patterns    = map {join '', map chr(97 + rand 26), 1 .. 5 } 1 .. 1
+00;
my @res         = map qr/$_/, @patterns;
my $whole_pat   = "@patterns";
my $whole_re    = qr/@patterns/;

cmpthese(-5, {
    'inline'        => sub {$target =~ /@patterns/},
    'inline-o'      => sub {$target =~ /@patterns/o},
    'grep_str'      => sub {return 1 if grep $target =~ $_, @patterns}
+,
    'grep_RE'       => sub {return 1 if grep $target =~ $_, @res},
    'whole_pat'     => sub {$target =~ /$whole_pat/},
    'whole_pat-o'   => sub {$target =~ /$whole_pat/o},
    'whole_re'      => sub {$target =~ $whole_re},
});

[download]
two sample outputs I get (unstable, given rand) is 
              Rate grep_str grep_RE inline inline-o whole_pat-o whole_
+pat whole_re
grep_str    96.6/s       --     -2%   -67%     -67%        -67%      -
+67%     -67%
grep_RE     99.1/s       3%      --   -67%     -67%        -67%      -
+67%     -67%
inline       296/s     207%    199%     --      -0%         -0%       
+-0%      -0%
inline-o     296/s     207%    199%     0%       --         -0%       
+-0%      -0%
whole_pat-o  297/s     207%    199%     0%       0%          --       
+-0%      -0%
whole_pat    297/s     207%    200%     0%       0%          0%       
+ --       0%
whole_re     297/s     207%    200%     0%       0%          0%       
+ 0%       --

[download]
              Rate grep_str grep_RE inline inline-o whole_re whole_pat
+ whole_pat-o
grep_str    97.5/s       --     -2%   -94%     -94%     -94%      -94%
+        -94%
grep_RE     99.8/s       2%      --   -94%     -94%     -94%      -94%
+        -94%
inline      1686/s    1628%   1589%     --      -0%      -1%       -1%
+         -1%
inline-o    1688/s    1630%   1591%     0%       --      -1%       -1%
+         -1%
whole_re    1707/s    1650%   1610%     1%       1%       --       -0%
+         -0%
whole_pat   1707/s    1650%   1610%     1%       1%       0%        --
+         -0%
whole_pat-o 1707/s    1650%   1610%     1%       1%       0%        0%
+          --

[download]
The list lengths were chosen so that the likely hood of actually getting a hit is reasonable (~80%). If we increase the pattern lengths to 10 characters so that failure is almost guaranteed, I get the following: 
             Rate grep_str grep_RE inline inline-o whole_pat-o whole_p
+at whole_re
grep_str    169/s       --     -4%   -46%     -46%        -46%      -4
+6%     -46%
grep_RE     177/s       5%      --   -43%     -43%        -44%      -4
+4%     -44%
inline      312/s      85%     77%     --      -0%         -0%       -
+0%      -0%
inline-o    312/s      85%     77%     0%       --         -0%       -
+0%      -0%
whole_pat-o 313/s      85%     77%     0%       0%          --       -
+0%      -0%
whole_pat   313/s      86%     77%     0%       0%          0%        
+--       0%
whole_re    313/s      86%     77%     0%       0%          0%        
+0%       --

[download]

You get negligible impact from the optimization, and you break your ability to update your array of patterns (potential for bugs). You also potentially confuse people less-sophisticated people who might look at your code. There is a clear improvement over using grep, but if benchmarking shows this step is your bottleneck, then you are probably better off either optimizing your pattern or rethinking your filtering.

#11929 First ask yourself `How would I do this without a computer?' Then have the computer do it the same way.

In reply to Re^3: More than one pattern match using grep on a file by kennethk
in thread More than one pattern match using grep on a file by vinoth.ree

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