Regex to match a username

Question

I am trying to create a regex to validate usernames which should match the following :

• Only one special char (._-) allowed and it must no

Answer 1

You should split your regex into two parts (not two Expressions!) to make your life easier:

First, match the format the username needs to have: ^[a-zA-Z][a-zA-Z0-9]*[._-]?[a-zA-Z0-9]+$

Now, we just need to validate the length constraint. In order to not mess around with the already found pattern, you can use a non-consuming match that only validates the number of characters (its literally a hack for creating an and pattern for your regular expression): (?=^.{3,20}$)

The regex will only try to match the valid format if the length constraint is matched. It is non-consuming, so after it is successful, the engine still is at the start of the string.

so, all together:

 (?=^.{3,20}$)^[a-zA-Z][a-zA-Z0-9]*[._-]?[a-zA-Z0-9]+$

Debugger Demo

Answer 2

I think you need to use ? instead of +, so the special character is matched only once or not.

^(?=(?![0-9])?[A-Za-z0-9]?[._-]?[A-Za-z0-9]+).{3,20}