[Admin]

In register.php and member.php replace this:

    $$varname=censortext($$varname);
    // ENTER ANY CUSTOM FIELD VALIDATION HERE!

With this:

    $$varname=censortext($$varname);
	if ($profilefield['profilefieldid'] == XX and preg_match('#[0-9]{10}#', $$varname)) {
		$$varname = '('.substr($$varname, 0, 3).') '.substr($$varname, 3, 3).'-'.substr($$varname, 6);
	}
    // ENTER ANY CUSTOM FIELD VALIDATION HERE!

(Without the spaces; quote this message to see the real source)