Beefy Boxes and Bandwidth Generously Provided by pair Networks
Pathologically Eclectic Rubbish Lister
 
PerlMonks  

Explained: Trying to count the captures in a compiled regular expression

by Roy Johnson (Monsignor)
on May 03, 2004 at 15:16 UTC ( #350042=note: print w/replies, xml ) Need Help??


in reply to Re: Re: Trying to count the captures in a compiled regular expression
in thread Trying to count the captures in a compiled regular expression

Upon a successful match, the match operator returns a list of the captures (or, in scalar context, the number of captures), one element for each set of capturing parentheses, even if the captured value is empty. By making the entire pattern optional, we ensure a successful match, and thus Perl will tell us how many groupings were.

As was pointed out by ysth and hugo, the case of no groupings will not return zero (because we need a true value to indicate a successful match), so we ought to put capturing parentheses around the expression, and subtract one from the result. And the possibility of embedded code, which we wouldn't want to run, is another caveat, so we want to use minimal matching. Hence:

my $regex = qr/foo/; $_ = 'anything'; my $matches = (() = /($regex)??/) - 1; # oops! fixed print "There were $matches groupings\n";

The PerlMonk tr/// Advocate
  • Comment on Explained: Trying to count the captures in a compiled regular expression
  • Download Code

Replies are listed 'Best First'.
Re: Explained: Trying to count the captures in a compiled regular expression
by ysth (Canon) on May 03, 2004 at 16:13 UTC
    That runs in scalar context, so won't work; try:
    $matches = (() = /($regex)??/) - 1;
      or even
      /$regex??/; print "There were $#+ groupings\n";

      The PerlMonk tr/// Advocate

Log In?
Username:
Password:

What's my password?
Create A New User
Node Status?
node history
Node Type: note [id://350042]
help
Chatterbox?
[dazz]: Hi I have grabbed an image from an IP camera with Image::Grab. I then want to pass that image object to a subroutine where I do a read-only test on the image with Image:Magick.
[dazz]: How do I pass an image object to a subroutine and then apply a Image::Magick a read-only method to the image object???

How do I use this? | Other CB clients
Other Users?
Others chilling in the Monastery: (9)
As of 2017-03-27 07:35 GMT
Sections?
Information?
Find Nodes?
Leftovers?
    Voting Booth?
    Should Pluto Get Its Planethood Back?



    Results (317 votes). Check out past polls.