Question

I am writing a PHP validation for a user registration form. I have a function set up to validate a username which uses perl-compatible regular expressions. How can I edit it so that one of the requirements of the regular expression are AT MOST a single . or _ character, but NOT allow that character at the beginning or end of the string? So for example, things like “abc.d”, “nicholas_smith”, and “20z.e” would be valid, but things like “abcd.”, “a_b.C”, and “_nicholassmith” would all be invalid.

This is what I currently have but it does not add in the requirements about . and _ characters.

function isUsernameValid()
{
    if(preg_match("/^[A-Za-z0-9_\.]*(?=.{5,20}).*$/", $this->username))
    {
        return true; //Username is valid format
    }
    return false;
}

Thank you for any help you may bring.

Answer 1

if (preg_match("/^[a-zA-Z0-9]+[._]?[a-zA-Z0-9]+$/", $this->username)) {
    // there is at most one . or _, and it's not at the beginning or end
}

You can combine this with the string length check:

function isUsernameValid() {
    $length = strlen($this->username);
    if (5 <= $length && $length <= 20
    &&  preg_match("/^[a-zA-Z0-9]+[._]?[a-zA-Z0-9]+$/", $this->username)) {
        return true;
    }
    return false;
}

You could probably do the whole lot using just one regex, but it would be much harder to read.