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

Using shouldChangeCharactersInRange screws up the pop-up key board as the backspace button doesn't work.

Number formatter is the way to go. Here's a sample I used to find positive decimals. I call it during the validation check- for e.g. when the user clicks on the save button.

    -(BOOL) isPositiveNumber: (NSString *) numberString {

    NSNumberFormatter *numberFormatter = [[NSNumberFormatter alloc] init];
    [numberFormatter setNumberStyle:NSNumberFormatterDecimalStyle];

    NSNumber *aNumber  =  [numberFormatter numberFromString:numberString];
    [numberFormatter release];

    if ([aNumber floatValue] > 0) {
        NSLog( @"Found positive number %4.2f",[aNumber floatValue] );

        return YES;
    }
    else {
        return NO;
    }
}