my $str="McGlaughlin http://www.karayiannis.com and http://www.samy.com";
$str =~ s/\b((?:http\:\/\/)|(?:www\.))([^ ]+)/<a href=\"$&\">$&<\/a>/sgi;
print $str;
__END__
McGlaughlin <a href="http://www.karayiannis.com">http://www.karayiannis.com</a> and <a href="http://www.samy.com">http://www.samy.com</a>