OnionKnight has asked for the wisdom of the Perl Monks concerning the following question:

Can you have a RegExp return the index where it found the match in a string? Just like the index() function, except with a regex. I know Javascript can.

2005-09-10 Retitled by holli, as per Monastery guidelines
Original title: 'RegExps'

  • Comment on Finding the index of a string with a regexp

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Re: Finding the index of a string with a regexp
by sk (Curate) on Sep 09, 2005 at 23:21 UTC
     perldoc -f pos

      pos SCALAR
       pos     Returns the offset of where the last "m//g" search 
               left off for the variable in question ($_ is used 
               when the variable is not specified).
               May be modified to change that offset.  Such 
               modification will also influence the "\G" zero-width 
               assertion in regular expressions.  See
               perlre and perlop.

    [download]

    here is an example -

    #!/usr/bin/perl -w
my $state = "mississippi";
while ($state =~ /.i/g) {
    print pos($state),":",$&,$/;
}
__END__
2:mi
5:si
8:si
11:pi

    [download]

    hope this helps!

    cheers

    SK

[reply]
[d/l]
[select]
      That's what I'm looking for, but isn't there any way to get it to return the offset for the beginning of the match instead of the end?
[reply]
        Look at the arrays @- and @+, which point to the start resp. the end of submatches — use index 0 for the whole match. So you want $-[0].

        Plus: unlike pos, it works without using /g, too.

        You didn't really think Javascript could do something with regexps that Perl couldn't, did you? Where did you think its implementors got the ideas from, anyway?

[reply]
[d/l]

        The first thing that comes to mind is substracting the length of the match, like in:

        pos ($state) - length $&

        --
        David Serrano

[reply]
[d/l]

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