Count capturing parentheses in a compiled regexp

Description:

This subroutines count the number of capturing parentheses in a compiled regular expression. Since the regexp is compiled, we know it's correct, and thus we only have to count the opening parentheses.

Update: added /s in the while condition.

Update: This other discussion led to a much better solution, so I commented out the original code.

#sub captures {
#    local $_ = shift;
#    croak "$_ is not a compiled regexp" unless ref eq 'Regexp';
#    my $n = 0;
#    while( /\G(?=.)/gcs ) {
#      /\G[^\\(]*/gc;     # ignore uninteresting stuff
#      /\G(?:\\.)*/gc;    # ignore backslashed stuff
#      /\G\(\?/gc;        # ignore special regexps
#      /\G\(/gc && $n++;  # a capturing (, count it!
#    }
#    $n;
#}

sub captures { ( @_ = '' =~ /(@{[shift]})??/ ) - 1; }

Comment on Count capturing parentheses in a compiled regexp Download Code

Replies are listed 'Best First'.
Re: Count capturing parentheses in a compiled regexp by hv (Prior) on May 02, 2004 at 12:12 UTC
Nice snippet, but a couple of problems: the outer lookahead needs to be //s, else eg: `qr{( x )}x;` [download] will fail. Also, this will find parens in embedded code and comments and treat as captures. If that doesn't seem worth worrying about it'd be enough to add a caveat I guess, else I think you can mimic perl's simplistic parsing reasonable easily for the code (just count to the balancing close-brace). Comments may actually be the trickiest, since you'll need to know when //x is in force: `qr{ (?x: # (comment) ) (?-x: # (capture) ) }` [download] Oops, another one: parens in `[ ... ]` should be ignored too; I'm not sure how easy those would be to parse, since not every `]` closes the selection. Hugo	[reply] [d/l] [select]
Re: Re: Count capturing parentheses in a compiled regexp by BooK (Curate) on May 02, 2004 at 15:20 UTC
Thanks a lot for finding these shortcomings in my code. `:-)` I'll submit updated versions as I correct them.	[reply]
Re: Count capturing parentheses in a compiled regexp by japhy (Canon) on May 02, 2004 at 16:03 UTC
Once I get Regexp::Parser working (it's the update to YAPE::Regex), you'll be able to do this via: `use Regexp::Parser; # pushes itself to @Regexp::ISA print qr/.../->nparens;` [download] _____________________________________________________ Jeff`[japhy]`Pinyan: Perl, regex, and perl hacker, who'd like a job (NYC-area) `s++=END;++y(;-P)}y js++=;shajsj<++y(p-q)}?print:??;`	[reply] [d/l]
Re: Count capturing parentheses in a compiled regexp by japhy (Canon) on Jun 27, 2004 at 03:35 UTC
In the spirit of "match the regex to get the number of parens in it", here's another way: `sub nparens { "" =~ /\|$_[0]/ and $#+ }` [download] _____________________________________________________ Jeff`[japhy]`Pinyan: Perl, regex, and perl hacker, who'd like a job (NYC-area) `s++=END;++y(;-P)}y js++=;shajsj<++y(p-q)}?print:??;`	[reply] [d/l]


XP is just a number
	PerlMonks