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counting matches


  Maintainer: Richard Proctor <richard@waveney.org>
  Date: 16 Aug 2000
  Last Modified: 20 Sep 2000
  Mailing List: perl6-language-regex@perl.org
  Number: 110
  Version: 6
  Status: Frozen


Provide a simple way of giving a count of matches of a pattern.


Have you ever wanted to count the number of matches of a patten? s///g returns the number of matches it finds. m//g just returns 1 for matching. Counts can be made using s//$&/g but this is wastefull, or by putting some counting loop round a m//g. But this all seams rather messy.

TomC (and a couple of others) have said that it can also be done as : $count = () = $string =~ /pattern/g;

However many people do not like this construct, here are a couple of quotes:

jhi: Which I find cute as a demonstration of the Perl's context concept, but ugly as hell from usability viewpoint.

Bart Lateur: '()=' is not perfect. It is also butt ugly. It is a "dirty hack".

This construct is also likely to be inefficient as perl will have to build up a list of all the matches, store them somewhere, count them, then throw them away.

Therefore I would like a way of counting matches.


m//gt (or m//t see below) would be defined to do the match, and return the count of matches, this leaves all existing uses consistent and unaffected. /t is suggested for "counT", as /c is already taken.

Relationship of m//t and m//g - there are three possibilities, my original:

m//gt, where /t adds counting to a group match (/t without /g would just return 0 or 1). However \G loses its meaning.

The Alternative By Uri :

m//t and m//g are mutually exclusive and m//gt should be regarded as an error.


> I like this too. I'd suggest /t should mean a) return a scalar of > the number of matches and b) don't set any special variables. Then > /t without /g would return 0 or 1, but be faster since no extra > information need be captured (except internally for (.)\1 type > matching - compile time checks could determine if these are needed, > though (?{..}) and (??{..}) patterns would require disabling of > that optimisation). /tg would give a scalar count of the total > number of matches. \G would retain its meaning.

I think Hugo's wording about the relationship makes the best sense, and this is the suggested way forward.


RFC110 V1 - Original posting to perl6-language

RFC110 V2 - Reposted to perl6-language-regex

RFC110 V3 - Added Uri's alternitive m//t

RFC110 V4 - Added notes about $count = () = $string =~ /pattern/g

RFC110 V5 - Added Hugo's wording about /g and /t relationship, suggested this is the way forward.

RFC110 V6 - Frozen


Hugo: > Implementation should be fairly straightforward, > though ensuring that optimisations occurred precisely when they > are safe would probably involve a few bug-chasing cycles.


I brought this up on p5p a couple of years ago, but it was lost in the noise...