Regex to match a username
I am trying to create a regex to validate usernames which should match the following :
- Only one special char
(._-)allowed and it must not be at the extremas of the string - The first character cannot be a number
- All the other characters allowed are letters and numbers
- The total length should be between 3 and 20 chars
This is for a HTML5 validation pattern, so sadly it must be one big regex.
So far this is what I've got:
^(?=(?![0-9])[A-Za-z0-9]+[._-]?[A-Za-z0-9]+).{3,20}
But the positive lookahead can be repeated more than one time allowing to be more than one special character which is not what I wanted. And I don't know how to correct that.
Answer
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]+$
