iOS 判断单个NSString是不是字母

//判断是不是小写字母
- (BOOL)isLowerLetter:(NSString *)str
{
    if ([str characterAtIndex:0] >=
'a' && [str characterAtIndex:0] <=
'z') {
        return YES;
    }
    return NO;
}

//判断是不是大写字母
- (BOOL)isCatipalLetter:(NSString *)str
{
    if ([str characterAtIndex:0] >=
'A' && [str characterAtIndex:0] <=
'Z') {
        return YES;
    }
    return NO;
}