You probably thought that (?<=199\d|200\d|\D) (note: the (?:) is useless here) would do something like (?:(?<=199\d)|(?<=200\d)|(?<=\D)), but that is not how it works. In pseudo-code, this is what the regex engine does:

# (?<=199\d|200\d|\D)

sub lookbehind {
    my $subregex = shift;  # qr/199\d|200\d|\D/;
    local pos() = pos() - matchlength($subregex);
    return /$subregex/;
}
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But here, the imaginary matchlength function can't return a single value, because the subregex can match strings of different lengths. So in your case, the engine doesn't know how many characters to go back before trying the look-behind.

Juerd
- http://juerd.nl/
- spamcollector_perlmonks@juerd.nl (do not use).