[PHP] Allowing punctuation marks in regular expressions

[Luis Lebron]

I found the following function to test a string for alphanumeric characters

function is_alphanumeric($string) 
{
        return (preg_match("/^[a-z0-9]+$/i", $string));
}

How can I modify it so that it also allows punctuation marks?

thanks,

Luis R. Lebron