取出一个字符串中的所有数字组成一个新的字符串

NSMutableString *mutStr = [NSMutableString string];
        
        for (int i=0; i<s.length; i++) {
            NSString *a = [s substringToIndex:i+1];
            a = [a substringFromIndex:i];
            [mutArrShu addObject:a];
        }
        
        for (NSString *b in mutArrShu) {
            NSString *emailRegex = @"^[0-9]*$";
            NSPredicate *emailTest = [NSPredicate predicateWithFormat:@"SELF MATCHES %@", emailRegex];
            BOOL isShu = [emailTest evaluateWithObject:b];
            if (isShu) {
                [mutStr appendString:b];
            }
        }