REG-EXPR: Allowing limited number of special chars in usernames

[Holger Sunke <hsunke@xxxxxx>]

Hello,

I dont know that much aboput regular expressions and just want to know how to 
find out the number of special (non alphanumeric) characters in a string or how to match a string that contains less than 3 special chars.

urrently im using a function

function valid_username($string) {
   if(strlen($string)<=16&&strlen($string)>5) {
      return(preg_match('/^[ -~äöüßÄÖÜ]+$/',$string));
   } else {
      return FALSE;
   }
}

This allows usernames of 6 to 16 length containing many special chars and German "Umlaute".
But this also allows usernames like "=)/(&%$" what should not be.

So how to limit the number of special chars? This function should just return fals if number of special chars > X

Greets,

Holger Sunke

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