Actually, lookbehind and lookahead seem to be the same when the assertion itself has zero-width... In the following code, looking ahead for the anchor produces the same results as looking behind for it.

#!/usr/bin/perl -wT
use strict;

# Replace 'See' anywhere in the string
$_ = "See spot run.  See spot jump.";
s/See/MATCH/g;
print "'$_'\n";          # 'MATCH spot run.  MATCH spot jump.'

# Replace 'See' unless lookahead finds the anchor
$_ = "See spot run.  See spot jump.";
s/(?!^)See/MATCH/g;
print "'$_'\n";          # 'See spot run.  MATCH spot jump.'

# Replace 'See' unless lookabehind finds the anchor
$_ = "See spot run.  See spot jump.";
s/(?<!^)See/MATCH/g;     # 'See spot run.  MATCH spot jump.'
print "'$_'\n";
[download]

-Blake