Checking String for illegal characters using regular expression

Question

I want to check a for any illegal character using the following regular expression in PHP. Essentially, I want to allow only alphanumeric and underscore (_). Unfortunately the

Answer 1

If $username only has alphanumeric and underscore it will return TRUE

if (preg_match("/^[a-z0-9_]+$/i", $username) )
{
    return true;
}