Both /(?=abc)\b/ and /(?<=abc)\b/ match the string "---abc", but not at the same position, as the program below shows:

#!/usr/bin/perl

use strict;
use warnings;

$_ = "---abc";

/(?=abc)\b/  and print  "Lookahead \$` = '$`'; \$' = '$''\n";
/(?<=abc)\b/ and print "Lookbehind \$` = '$`'; \$' = '$''\n";

__END__
Lookahead $` = '---'; $' = 'abc'
Lookbehind $` = '---abc'; $' = ''
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/(?=abc)\b/ matches at a word boundary that is followed by "abc", hence after "---", but before "abc". /(?<=abc)\b/ matches at a word boundary that is preceded by "abc", hence after "abc", and right before the end of the string.

Abigail