Answer by drewish for What's the best way to validate currency input in UITextField?

I found this to be a relatively clean approach. I haven't tested it with non-US currencies but since it uses the NSNumberFormatter's properties I believe it should handle them correctly.

Start by setting up a formatter some place:

    formatter = [NSNumberFormatter new];
    [formatter setNumberStyle: NSNumberFormatterCurrencyStyle];
    [formatter setLenient:YES];
    [formatter setGeneratesDecimalNumbers:YES];

Use the formatter to parse and reformat their input. It also handles shifting the number when digits are added and removed.

-(BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string
{
    NSString *replaced = [textField.text stringByReplacingCharactersInRange:range withString:string];
    NSDecimalNumber *amount = (NSDecimalNumber*) [formatter numberFromString:replaced];
    if (amount == nil) {
        // Something screwed up the parsing. Probably an alpha character.
        return NO;
    }
    // If the field is empty (the initial case) the number should be shifted to
    // start in the right most decimal place.
    short powerOf10 = 0;
    if ([textField.text isEqualToString:@""]) {
        powerOf10 = -formatter.maximumFractionDigits;
    }
    // If the edit point is to the right of the decimal point we need to do
    // some shifting.
    else if (range.location + formatter.maximumFractionDigits >= textField.text.length) {
        // If there's a range of text selected, it'll delete part of the number
        // so shift it back to the right.
        if (range.length) {
            powerOf10 = -range.length;
        }
        // Otherwise they're adding this many characters so shift left.
        else {
            powerOf10 = [string length];
        }
    }
    amount = [amount decimalNumberByMultiplyingByPowerOf10:powerOf10];

    // Replace the value and then cancel this change.
    textField.text = [formatter stringFromNumber:amount];
    return NO;
}