+------------ Matched at pos 0.
           |   +-------- Matched at pos 1.
           |   |+------- Matched at pos 2.
           |   || +----- Matched at pos 1.
           |   || |+---- Matched at pos 2.
           |   || ||+--- Matched at pos 3.
           |   || |||
           v   vv vvv
'abcd' =~ /a(?!BC)..d/  # Matches

##</code><code>##

my $s = 'abcd';
$s =~ s/a(?=bc)/x/;  # xbcd

my $s = 'aefg';
$s =~ s/a(?=bc)/x/;  # abcd