function rC(fld) {
	var allowedChars = /[c]/;
    if (!allowedChars.test (fld.value)){
      fld.value = fld.value.replace (/([^c])/g, "");
      alert("Only 'c' is allowed in this field.");
  }
};
function rX(fld) {
	var allowedChars = /[x]/;
    if (!allowedChars.test (fld.value)){
      fld.value = fld.value.replace (/([^x])/g, "");
      alert("Only 'x' is allowed in this field.");
  }
};