If  $r is really just a plain, simple string of characters, it seems to me you're asking questions that can easily be answered by index. If  $r is a regex of limitless complexity, it seems much more difficult, perhaps impossible, to answer the question "could there be a match if more characters were added to the right end of $s?" 

c:\@Work\Perl>perl -wMstrict -le
"my $r = '123';
 ;;
 for my $s ('', qw(1 12 123 1234 2 23 234)) {
   my $left_same = 0 == index($r, $s);
   my $all_same  = $r eq $s;
   printf qq{%-6s with more stuff could %smatch with '$r' \n},
          qq{'$s'}, ($left_same and not $all_same) ? '' : 'NOT '
          ;
   }
"
''     with more stuff could match with '123'
'1'    with more stuff could match with '123'
'12'   with more stuff could match with '123'
'123'  with more stuff could NOT match with '123'
'1234' with more stuff could NOT match with '123'
'2'    with more stuff could NOT match with '123'
'23'   with more stuff could NOT match with '123'
'234'  with more stuff could NOT match with '123'

[download]
Updates:
  1. Maybe the idea of Levenshtein distance (LD) could also be brought to bear. I.e., if the two strings (again, I'm assuming they're both just simple strings) are the same at the left, the LD gives an idea of how much one must change the shorter string to make it the same as the longer. (LD == 0 means both strings were exactly the same to begin with.)
  2. OTOH, If you just want to answer the question "can  $s with one or more characters added and anchored at the start of  $r match?" (again assuming  $s $r to be plain strings), the regex
        $r =~ m{ \A \Q$s\E .+ }xms
    would seem to do the trick (note reversal of $s and $r).

In reply to Re: Can I determine index point of failure in string during regex match attempt by AnomalousMonk
in thread Can I determine index point of failure in string during regex match attempt by tj_thompson

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