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

my $test = "ASDFE:asdfe";
$test =~ s!             # Substitute
            (?<=        # Zero-width positive lookbehind
              [A-Z]{5}  # Five caps
            )           # end lookbehind
            :           # Substituting a colon (but not the preceeding characters)
          !:</B>\n<BR><BR>!x;
print $test;