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".

Answer 2

Not ideal, but easy: Use a second pass with the regex /^.*[a-zA-Z_\-].*$/

Just ensure it passes both and you'll be fine.

Answer 3

If you can go with two passes, a simpler and faster second pass regexp is:

/[^\d]/

This just matches anything that is not a number and it needs to match only one and it terminates early. You don't really need to be strict here because the first pass already rejects non-allowable characters.

Answer 4

I would use the regex that you need for validation and then something like:

passwd.to_i.to_s.length != passwd.length

to verify that passwd is not a string of digits after it passes the primary validation.

Answer 5

Yet another way, though it may not perform as well as Max's:

/^[a-z0-9][-\w]*[-_a-z][-\w]*$/i