static void Main(string[] args)
        {

            var a = "TESTE :P0 :P1 :P12 :P13 :P2 :P21";

            var matches = Regex.Matches(a, ":P[0-9]+($|\\s)");

            foreach (var match in matches)
            {
                a = a.Replace(match.ToString(), "'PPPP' ");
            }

        }