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        $str .= $1 . $/ while ( $ARGV[0] =~ m/(.{3})/g );
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use constant LEN => 3;
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            $offset += $length;
        }
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#!usr/bin/perl -w
use strict;
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);

cmpthese($re);

Benchmark: running regex, substring for at least 2 CPU seconds...
     regex:  1 wallclock secs ( 2.09 usr +  0.00 sys =  2.09 CPU) @ 14
+030.14/s (n=29323)
...
             Rate     regex substring
regex     14030/s        --      -40%
substring 23226/s       66%        --

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        $str .= $1 . $/ while ( $ARGV[0] =~ m/(.{3})/g ); ## NOTE the 
+number 3
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        $str .= $1 . $/ while ( $ARGV[0] =~ m/(.{LEN})/g ); ## NOTE 3 
+is now LEN
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Benchmark: running regex, substring for at least 2 CPU seconds...
     regex:  4 wallclock secs ( 2.72 usr +  0.01 sys =  2.73 CPU) @ 74
+0317.95/s (n=2021068)
...
              Rate substring     regex
substring  17333/s        --      -98%
regex     740318/s     4171%        --