If \b, a zero-width assertion, doesn't care whether it's placed before or after the word it's guarding (so to speak), why should (?= ) care that it is for lookahead only? What is it that makes (?<= ) necessary, and (?= ) unable to fill both niches?

You seem to be under the assumption that (?= ) and (?<= ) match the same thing. That isn't true, as the program below shows: 

#!/usr/bin/perl

use strict;
use warnings;
no warnings qw /syntax/;

$_ = "---abc";

print "Lookahead matches\n"  if /(?=abc)\w/;
print "Lookbehind matches\n" if /(?<=abc)\w/;

__END__
Lookahead matches

[download]

See, if (?= ) and (?<= ) are different, no construct could replace them both.

Abigail

In reply to Re: Why do zero width assertions care about lookahead/behind? by Abigail-II
in thread Why do zero width assertions care about lookahead/behind? by davido

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