It's sometimes useful to know where in a string a match occurs.

c:\@Work\Perl\monks>perl -wMstrict -le
"my $s = 'kkkaaabc';
 for my $rx (
   'k?a?', 'k*a?', 'k?a+', 'k+a+', 'k?a*', 'k*a*', 'a*',
   ) {
   print qq{'$s'};
   ;;
   $s =~ m{ ($rx) }xms;
   ;;
   if (not defined $1) {
     print 'no match';
     next;
     }
   ;;
   print ' ', ' ' x $-[1], '^' x ($+[1] - $-[1]),
         qq{  /$rx/ matched '$1' at offset $-[1]};
   }
"
'kkkaaabc'
 ^  /k?a?/ matched 'k' at offset 0
'kkkaaabc'
 ^^^^  /k*a?/ matched 'kkka' at offset 0
'kkkaaabc'
   ^^^^  /k?a+/ matched 'kaaa' at offset 2
'kkkaaabc'
 ^^^^^^  /k+a+/ matched 'kkkaaa' at offset 0
'kkkaaabc'
 ^  /k?a*/ matched 'k' at offset 0
'kkkaaabc'
 ^^^^^^  /k*a*/ matched 'kkkaaa' at offset 0
'kkkaaabc'
   /a*/ matched '' at offset 0
[download]

Note in particular the  /a*/ regex which I added at the end. This matches the empty string at offset 0 even though a perfectly good  'aaa' sequence is available further on. Engrave "Leftmost Longest" on a prayer wheel and keep it ever spinning in your mind.

Regexes are the most counterintuitive thing I've encountered in the realm of programming.

Updates:

1. The initialization of capture variables (e.g., $1) to undef on regex recompilation is apparently only available from Perl version 5.10 onward. The code

   my $match = $s =~ m{ ($rx) }xms;
   
   if (not $match) {
       print 'no match';
       next;
       }
   [download]

   is more portable among different Perl versions.
2. Also check out davido's Perl Regular Expression Tester

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