problem with making preg_match for a username validating system

Question

I made this regex code:

/^([a-zA-Z\d]+[_]?+[a-zA-Z\d]){3,12}$/

I want the expression to:

• only allow it to start/end with any letter or digit
• allow a under score in the middle
• only 3 to 12 chars

When I test a name with more than 12 chars it still gives me a positive result.

How do I fix this?

Answer 1

You may use

^(?=.{3,12}$)[a-zA-Z\d]+(?:_[a-zA-Z\d]+)?$

Details

• ^ - start of string
• (?=.{3,12}$) - length allowed from 3 to 12 chars
• [a-zA-Z\d]+ - 1+ letters or digits
• (?:_[a-zA-Z\d]+)? - an optional sequence of _ and 1+ digits/digits
• $ - end of string