Oh right. Well it was a quickie answer anyway (at least I got the explanation right). I'd avoid /g there though since the OP might accidentally call it in list context and screw it all up. I'd probably just do the later match against a substr() lvalue like my $end = $start + (substr( $str, $start ) =~ /b/ ? $-[0] : 0);. Roughly like that anyway. I didn't mention that I tested none of this - I'm just writing it and figuring that I know how to speak perl correctly.

I also have an aversion to pos() since I know that its behaviour is currently undefined with regard to local(). In this case that "bug" isn't relevant so I suppose I could go along with a use of pos(). Exactly what you'd do with it though... I dunno.

# Maybe this. I think I'd rather just access $+[0] directly.
my $start = $str =~ /a/g ? pos $str : 0;
my $end = $str =~ /b/g ? $-[0] : 0;
my $length = $end > $start ? $end - $start : 0;

my $ext = $length ? substr( $str, $start, $length ) : undef;
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