How about just using it, marked optional, and seeing how many captures you get? 
$_ = 'anything';
my @capture_count;
my $regex = qr/one(.*?)((two)(four))/;
@capture_count = /(?:$regex)?/;
print @capture_count." captures\n";

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In reply to Re: Trying to count the captures in a compiled regular expression by Roy Johnson
in thread Trying to count the captures in a compiled regular expression by BooK

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