function mascaraTexto(evento, mascara){  
     
  var campo, valor, i, tam, caracter;  
     
  if (document.all) // Internet Explorer  
  campo = evento.srcElement;  
  else // Nestcape, Mozzila  
  campo= evento.target;  
         
  valor = campo.value;  
  tam = valor.length; 
  
  for(i=0;i<mascara.length;i++){  
      caracter = mascara.charAt(i);  
     if(caracter!="9")   
         if(i<tam & caracter!=valor.charAt(i))  
             campo.value = valor.substring(0,i) + caracter + valor.substring(i,tam);                  
 }    
 }