preg_replace in PHP - regular expression for NOT condition

You can use a negated character class (using ^ at the beginning of the class):

/[^\da-z]+/i

Update: I mean, you have to use a negated character class and you can use the one I provided but there are others as well ;)

Try

preg_replace("/([^a-zA-Z0-9]+)/","*",$mystring);

You want to use a negated "character class". The syntax for them is [^...]. In your case just [^\w] I think.