Regex that expresses "at least one non-digit"

Question

I want to validate usernames according to this schema:

1. Allowable characters: letters, numbers, hyphen, underscore
2. First character must be a letter or a number
3. The username cannot be all numbers

This regular expression satisfies 1 and 2 above, but I can't figure out how to satisfy 3:

/^[a-zA-Z\d][\w\-]+$/

(I'm using Ruby, if that's relevant)

Answer 1

Not very efficient, but simple:

/^(?!\d+$)[a-zA-Z\d][\w\-]+$/

The lookahead simply means: "what follows isn't a string of numbers that go on until the end".