To do this you can use the natty library:
link
What would work for the 2 cases:
case a)
import com.joestelmach.natty.*;
List<Date> dates = new Parser().parse("Cod. principal: 785369-06 Fecha 12/10/2015").get(0).getDates();
System.out.println("Fecha: " + dates.get(0));
case b)
List<Date> dates = new Parser().parse("Fecha 2015-11-24 asddsa12 Otros datos: 12202").get(0).getDates();
System.out.println("Fecha: " + dates.get(0));
Even to search several dates in the same chain:
List<Date> dates = new Parser().parse("Cod. principal: 785369-06 Fecha 12/10/2015 Fecha 2015-11-24 asddsa12 Otros datos: 12202").get(0).getDates();
System.out.println("Fecha1: " + dates.get(0));
System.out.println("Fecha2: " + dates.get(1));
Another option is as comrades comment through split ().
String texto = "Cod. principal: 785369-06 Fecha 14/10/2015";
String[] array_split = texto.split("Fecha");
String myFecha = array_split[1].trim();