You're right, it shouldn't match an IP (unless you mean hostname?). Please provide some more sample input with comments on what should match and what shouldn't.

With the limited specs you've given, here's a regex: /^# ?(?:[A-Z]{2}|[a-z]{2})/ That'll match any two uppercase letters or any two lowercase letters, but not mixed ("Mo"). If you want mixed case to match too: /^# ?[A-Z]{2}/i If you want to match only two letters, not more, stick a \b at the end of the regex.