I use index when I need the match position, otherwise a regex. And it seems that index is NOT faster. Even code like 
  my $pos; 
  if ( $line =~ $regex ) { 
     $pos = length $`; 
  }

[download]
which gets the match position with a regex is slightly faster (but much uglier of course):

Update: For better ways of getting the match position, see How do I retrieve the position of the first occurrence of a match?.

Benchmark code:

#!/usr/bin/perl
use strict;
use warnings;
use Benchmark qw(:all) ;

my $count = 5000;

my $filename = 'TEST.dat';
my $DELIMITER = 'GGAGAGGG';
#my $DELIMITER = 'TTTTCATGAAGAAGATGAGAGACAAGATGAGAAAATAGTATCAGAGA';
my $regex = qr{\Q$DELIMITER}o;

 cmpthese($count, {
               'index' => sub { 
                   open my $FH, '<', $filename; 
                   my $i;
                   while (my $line = <$FH>) {
                       my $pos = index $line, $DELIMITER; 
                       if ( $pos >= 0 ) {
                           $i++;
                       }    
                   } 
                   close $FH;
               },
               'regex_compiled_pos' => sub {
                   open my $FH, '<', $filename;
                   my $i;
                   while (my $line = <$FH>) {
                       my $pos;
                       if ( $line =~ $regex ) {
                           $i++;
                           $pos = length $`;
                       }    
                   }  
                   close $FH;
               },
               'regex_compiled' => sub {
                   open my $FH, '<', $filename;
                   my $i;
                   while (my $line = <$FH>) {
                       if ( $line =~ $regex ) {
                           $i++; 
                       }    
                   }       
                   close $FH;
               },  
               'regex_pos' => sub {
           open my $FH, '<', $filename;
                   my $i;
                   while (my $line = <$FH>) {
                       my $pos;
                       if ( $line =~ /\Q$DELIMITER/ ) {
                           $i++;
                           $pos = length $`;
                       }
                   }
                   close $FH;
               },
               'regex' => sub {
                   open my $FH, '<', $filename;
                   my $i;
                   while (my $line = <$FH>) {
                       if ( $line =~ /\Q$DELIMITER/ ) {
                           $i++;
                       }
                   }
                   close $FH;
               },
           });

[download]
Benchmark results: 
                    Rate index regex_pos regex regex_compiled_pos rege
+x_compiled
index              450/s    --      -38%  -39%               -40%     
+      -41%
regex_pos          728/s   62%        --   -2%                -3%     
+       -5%
regex              741/s   65%        2%    --                -1%     
+       -3%
regex_compiled_pos 749/s   66%        3%    1%                 --     
+       -2%
regex_compiled     763/s   70%        5%    3%                 2%     
+        --

[download]

In reply to Re: Style question: regex versus string builtin function by lima1
in thread Style question: regex versus string builtin function by eyepopslikeamosquito

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