How to Test Character Input to UITextField as the User Enters Characters and Prevent Invalid Characters

Question

First, I setup up the keyboard for the UITextField to use the number with decimal style. So the user can only enter numbers and a single decimal.

What I want to do is test the input as the user enters it and prevent multiple decimals from being entered and limit the decimal portion of the number to two places. I do not want to round off the number nor even treat the input as a number. I simply want to prevent the user from entering more then two digits to the right of the decimal place.

Answer 1

The solution ultimately turned out to be fairly trivial. Unfortunately a lot of the questions and answers related to this question are about validating or formatting numeric values, not controlling what a user could input.

The following implementation of the shouldChangeCharactersInRange delegate method is my solution. As always, regular expressions rock in this situation. RegExLib.com is an excellent source for useful RegEx samples or solutions. I'm not a RegEx guru and always struggle a bit putting them together so any suggestions to improve it are welcome.

- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string
{
    if (textField == self.quantityTextField)
    {
        NSString *newString = [textField.text stringByReplacingCharactersInRange:range withString:string];

        NSString *expression = @"^([0-9]+)?(\\.([0-9]{1,2})?)?$";

        NSRegularExpression *regex = [NSRegularExpression regularExpressionWithPattern:expression 
                                                                               options:NSRegularExpressionCaseInsensitive 
                                                                                 error:nil];
        NSUInteger numberOfMatches = [regex numberOfMatchesInString:newString
                                                            options:0
                                                              range:NSMakeRange(0, [newString length])];        
        if (numberOfMatches == 0)
            return NO;        
    }

    return YES;
}

The above code allows the user to input these kinds of values: 1, 1.1, 1.11, .1, .11. Notice that this implementation does not replace the text field's string, which would causes a recursion. This solution simply rejects the next character the user inputs if the 'newString' does not match the regex.