How to validate or Allow Only Numeric value TextField in IOS or objective-c

– (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string
{
BOOL allowChange = YES;
if ([textField isEqual:zipText])
{
allowChange = NO;
int textLength = [textField.text length];
int value;
if (([string isEqualToString:@””]))
{
allowChange=YES;
}
else
{
value = (int) [string characterAtIndex:0];
}
if ((textLength <= kMaxZipLength) && value >=48 && value <=57)
{
allowChange=YES;
}
}

return allowChange;
}