I'm trying to use a zero-width "Look-Around Assertion", part of the extended regular expression syntax of Perl. Here's a little test code that isn't working as I expect: 
my $string = '54W';
if ($string =~ /(?=4)W/x) {
    warn "1 true\n";
}
if ($string =~ /(?=\d)W/x) {
    warn "2 true\n";
}
if ($string =~ /\dW/x) {
    warn "3 true\n";
}

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Only the 3rd regular expression shows as true. I don't understand why the '4' is not matching the (?=4) pattern or (?=\d) pattern in the first two tests. Regards, John

In reply to zero-width assertions for extended regular expressions by johnrcomeau

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