This is one the trickiest parts of any application at all. Why is that? Because the developer doesn't control it. You can write the best algorithm in the world, but still if it includes user input there is a place for mistakes. Even if we put some coplicated logic to prevent the input of wrong symbols, check the consistence of the data and do whatever possible to make sure that it is all OK, there is still possibility that the users enter the wrong number. Though all said, we must try to prevent the most of human errors and the best way to do this is by using Regular Expressions. Basicly Regular Expressions are used for string matches. They are based on search and pattern matching strings in text. A lot of books are written about them, there are even some programming languages designed especially for Regular Expressions.