CODEWITHSUNDEEP
regexperl
Greg Nisbet
Greg Nisbet

Reputation: 6994

Perl check whether string is prefix of valid regex match

Is it possible to determine, in general, whether a given string is a prefix of any possible match of a regex in Perl?

One interesting function that the OCaml regex library Str supports is string_partial_match. This function determines whether the portion of a string starting at a given position is either a) a prefix of a match or literally a match itself or b) not a prefix of any matching string.

From the Str documentation:

val string_partial_match : regexp -> string -> int -> bool

Similar to Str.string_match, but also returns true if the argument string is a prefix of a string that matches. This includes the case of a true complete match.

I was wondering if Perl has the ability to emulate this function. Using pos() to set the position, the zero-width assertion \G, and regex interpolation, we can emulate string_match (which doesn't have the prefix behavior).

sub string_match {
    my ($regex, $str, $pos) = @_;
    pos($str) = $pos;
    return ($str =~ m/\G$regex/);
}

Perl's regexes support many features that OCaml's library doesn't, so it's entirely possible that the ability to recognize prefixes of possible matches isn't possible to implement / isn't exposed by the API.

Upvotes: 1

Views: 906

Answers (1)

fishinear
fishinear

Reputation: 6336

If I understand you correctly, then you want to see whether the string could match the regular expression if the string would be longer. That is, whether it is possible to extend the string in a way that matches the regular expression. In technical terms: when the DFA reaches the end of the string it has not failed yet.

I can see some uses for such functionality, especially in verifying user input interactively one character at a time. Then you would like to distinguish between "the input will never match", and "the input could match if the user enters more characters".

As far as I know, though, such functionality is not supported by Perl.

Upvotes: 1

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