#define LEGAL_TEXT @"0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz "

- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string {
     NSCharacterSet *cs = [[NSCharacterSet characterSetWithCharactersInString:LEGAL_TEXT] invertedSet];
     NSString *filtered = [[string componentsSeparatedByCharactersInSet:cs] componentsJoinedByString:@""];
     return [string isEqualToString:filtered];
}

entryField.keyboardType = UIKeyboardTypeNumbersAndPunctuation;

#define NUMBERS	@"0123456789"
#define NUMBERSPERIOD	@"0123456789."

- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string
{
    NSCharacterSet *cs;
    NSString *filtered;

    // Check for period
    if ([entryField.text rangeOfString:@"."].location == NSNotFound)
    {
        cs = [[NSCharacterSet characterSetWithCharactersInString:NUMBERSPERIOD] invertedSet];
        filtered = [[string componentsSeparatedByCharactersInSet:cs] componentsJoinedByString:@""];
        return [string isEqualToString:filtered];
    }

    // Period is in use
    cs = [[NSCharacterSet characterSetWithCharactersInString:NUMBERS] invertedSet];
    filtered = [[string componentsSeparatedByCharactersInSet:cs] componentsJoinedByString:@""];
    return [string isEqualToString:filtered];
}