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This might not be as slick as the lookahead approaches described above, but in the spirit of TMTOWTDI here's a version that uses pos and the @- array, which contains the offset of the start of the last match (see perlvar for more info):

sub count_matches { my ( $pattern, $string ) = @_; my $num_matches = 0; while( $string =~ m/$pattern/gi ) { pos( $string ) = $-[0] + 1; $num_matches++ } return $num_matches; }

Update: For monks (like me) that didn't understand why a pattern consisting entirely of a zero-width lookahead assertion (m/(?=AA)/g, see the above responses) doesn't get stuck in an infinite loop, see perlre, "Repeated patterns matching zero-length substring". From that doc:

Perl allows such constructs, by forcefully breaking the infinite loop ... when Perl detects that a repeated expression matched a zero-length substring.

To break the loop, the following match after a zero-length match is prohibited to have a length of zero.

... the second best match is chosen if the best match is of zero length ... the second-best match is the match at the position one notch further in the string.
Thanks to ambrus for the pointer to the right section in the docs.

In reply to Re: counting overlapping patterns by bobf
in thread counting overlapping patterns by Anonymous Monk

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