http://www.perlmonks.org?node_id=424210


in reply to Things you think you'll never use.

How can we express the regular expression "some number of X's followed by the same number of Y's" in the most elegant way as a regex. I always thought it's quite problematic, until I saw your post.

Replies are listed 'Best First'.
Re: I wonder...
by Jenda (Abbot) on Jan 22, 2005 at 14:40 UTC

    You mean something like: 

    $s = "xxyyyyyy";

$s =~ m/((x+)(??{'y' x length($2)}))/ and print "$1\n";

    [download]
    ? :-)

    Jenda
    We'd like to help you learn to help yourself
    Look around you, all you see are sympathetic eyes
    Stroll around the grounds until you feel at home
       -- P. Simon in Mrs. Robinson

[reply]
[d/l]
      Yes this is exactly what I meant :-) But it's not the point of my post. The point was: this matching task is a real challenge for 'normal regexes'. This technique using ??{} is indeed a way to solve it. I wondered whether there are other ways, using more standard regex operators.
[reply]
Re: I wonder...
by Aristotle (Chancellor) on Jan 23, 2005 at 16:15 UTC

    If you want to match at least the same number, but it's okay to gobble up more:

    /(x+)(y+)(?(?{ length $2 < length $1 })(?!))/x

    [download]

    Make it != if both matches must be the exact same length.

    Makeshifts last the longest.

[reply]
[d/l]

In Section Meditations