Question

I’m trying to learn how to use preg_match. I want users to be only allowed to sign up with username between 2-20 characters which can contain a-zA-Z0-9.

Now the tricky part where Im getting lost, I want them to be able to include one hyphen anywhere in the username so,

-Brad = TRUE

--Brad = FALSE

B-Rad = TRUE

Answer 1

You can build this up step-by-step. You want a username that consist of 2-20 specific characters:

^[a-zA-Z0-9]{2,20}$

Now you want to allow a single - character somewhere in there (the trick part):

The - character is only allowed if it is not followed up by another - to the end of the string:

[-](?=[^-]*$)

This is a so called Lookahead assertion. Combined with an alternation, the regex completes to:

^(?:[a-zA-Z0-9]|[-](?=[^-]*$)){2,20}$

Compared to the other answers given, this one respects your length specification.