Hi.  regex hurts my brain.  This simple function will validate a username.

1. between 4 and 20 characters

2. can only be numbers or letters
 

function f_validate_username($username) {
    $normal = "^([a-zA-Z0-9]{4,20})$";
    if (eregi($normal, $username)) {
          return true;
    }
       else {
          return false;
    }
}

What I want to add, but can't figure out... is how to check to make sure it doesn't START with a number.

matches:  ab1cde , aBcd1, aaaa, a1111

doesn't match:   1acbd

Thanks :) 

Try:

^([a-zA-Z][a-zA-Z0-9]{3,19})$

Remember, the quantifier only applies to the immediately preceding item.

Susan