Hello again Monks!

In addition to my long question below, I've come up with a shorter more theoretical question that should be easier to jump into.

Given a string $s, and a regex $r anchored at both front and end of the string with '^' and '$', is it possible to write a function could_match( $r, $s ) that will determine whether $r could match if additional characters were appended to $s?

One requirement: the entirety of $s must be matched, not a sub string of $s. $r could always match with additional characters if it was not anchored.

It seems this should be easy enough to answer if you can answer the following question: How much of string $s was consumed in the failed match attempt against $r? If the entire string was consumed and $r remained unmatched, then it would seem $r could match if characters were appended to $s. On the other hand, if only part of $s was consumed, then adding additional characters would not help and $r could not be matched.

Perhaps consumed is poor verbiage. You could also look at this as the index point of failure during the match attempt.

Examples: 

# $s does not match, but '12' would be consumed in the 
# match attempt.  In this case, all of $s is consumed
# so $s should be able to match if additional characters
# were added
$s = '12'
$r = qr/^123$/;
could_match( $r, $s )  # returns TRUE

# $s does not match and none of $s would be consumed in the
# attempt as the '1' in the regex could not match.  Since
# $s was not consumed in it's entirety in the match attmempt
# there are no characters that could be added to $s to allow
# it to match $r
$s = '23'
$r = qr/^123$/;
could_match( $r, $s )  # returns FALSE

# $r in this case is not anchored, so it could always match
# if more characters were added.  However, since none of $s
# was used in the match attempt, the entirety of $s could
# not match even if more characters were added.
$s = '123';
$r = qr/456/;
could_match( $r, $s )  # returns FALSE

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

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