use strict;
use warnings;

while (<DATA>) {
    s|\b(\d{3,5})\b|<b><a href='lookup.cgi?id=$1</a></b>|g;
    print;
}

__DATA__
foo 300 bar bas.
foo300bar bas.
foo 123456 bar 1234 bas 56789.
foo bar bas.
1 foo 1 bar 2 bas bas.