regex for usernames

Question

I am trying to figure out how to setup a regex expression on doing the following requirements.

• Must consist at least two characters that are alpha characters a-zA-Z
• Must consist only ONE underscore or dash allowed anywhere AFTER the first check, the dash/underscore cannot be at the end as the same rule to apply as the first step
• Must be alpha-numeric characters.

Basically a good example is

Acceptable Usernames

• myusername_09
• username-09
• bw-09

Unacceptable Usernames

• bw 09
• bw--09
• bw_-09
• username__09

If any help would appreciate, this is what I had but its not working for me as what I want it to be.

^(?=[A-Za-z0-9])(?!.*[_-]{2})[A-Za-z0-9_-]+$

Answer 1

If I understand your requirement correctly, you just need to validate that the username is correct? If so, I'd use this regex:

^[A-Za-z]{2,}[_-]?[A-Za-z0-9]{2,}$

You did not say how many characters would be required after the dash or underscore; my example requires at least 2 more after. It can be altered as needed.

EDIT: I've added the ? after the [_-] to account for that being optional, per your comment below.

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Some additional information on quantifiers:

• {N,} means that there must be at least N characters from the preceding item to match.
• {N} means there must be exactly N of the preceding.
• {N,M} means there must be at least N, but no more than M.
• ? means there must be 0 or 1 of the preceding.
• + means there must be 1 or more.
• * means there must be 0 or more.