Task: IBAN
C++: #include <string>
#include <iostream>
#include <boost/algorithm/string.hpp>
#include <map>
#include <algorithm>
#include <cctype>

using namespace boost::algorithm;

bool isValid(const std::string &ibanstring) {
    static std::map<std::string, int> countrycodes {
        {"AL", 28}, {"AD", 24}, {"AT", 20}, {"AZ", 28 },
        {"BE", 16}, {"BH", 22}, {"BA", 20}, {"BR", 29 },
        {"BG", 22}, {"CR", 21}, {"HR", 21}, {"CY", 28 },
        {"CZ", 24}, {"DK", 18}, {"DO", 28}, {"EE", 20 },
        {"FO", 18}, {"FI", 18}, {"FR", 27}, {"GE", 22 },
        {"DE", 22}, {"GI", 23}, {"GR", 27}, {"GL", 18 },
        {"GT", 28}, {"HU", 28}, {"IS", 26}, {"IE", 22 },
        {"IL", 23}, {"IT", 27}, {"KZ", 20}, {"KW", 30 },
        {"LV", 21}, {"LB", 28}, {"LI", 21}, {"LT", 20 },
        {"LU", 20}, {"MK", 19}, {"MT", 31}, {"MR", 27 },
        {"MU", 30}, {"MC", 27}, {"MD", 24}, {"ME", 22 },
        {"NL", 18}, {"NO", 15}, {"PK", 24}, {"PS", 29 },
        {"PL", 28}, {"PT", 25}, {"RO", 24}, {"SM", 27 },
        {"SA", 24}, {"RS", 22}, {"SK", 24}, {"SI", 19 },
        {"ES", 24}, {"SE", 24}, {"CH", 21}, {"TN", 24 },
        {"TR", 26}, {"AE", 23}, {"GB", 22}, {"VG", 24 }
    };
    std::string teststring(ibanstring);
    erase_all(teststring, " "); 
    if (countrycodes.find(teststring.substr(0, 2)) == countrycodes.end())
        return false;
    if (teststring.length() != countrycodes[teststring.substr(0, 2)])
        return false;
    if (!all(teststring, is_alnum()))
        return false;
    to_upper(teststring);
    std::rotate(teststring.begin(), teststring.begin() + 4, teststring.end());

    std::string numberstring; 
    for (const auto &c : teststring) {
        if (std::isdigit(c))
            numberstring += c;
        if (std::isupper(c))
            numberstring += std::to_string(static_cast<int>(c) - 55);
    }
    
    
    
    int segstart = 0;
    int step = 9;
    std::string prepended;
    long number = 0;
    while (segstart < numberstring.length() - step) {
        number = std::stol(prepended + numberstring.substr(segstart, step));
        int remainder = number % 97;
        prepended = std::to_string(remainder);
        if (remainder < 10)
            prepended = "0" + prepended;
        segstart = segstart + step;
        step = 7;
    }
    number = std::stol(prepended + numberstring.substr(segstart));
    return (number % 97 == 1);
}

void SayValidity(const std::string &iban) {
    std::cout << iban << (isValid(iban) ? " is " : " is not ") << "valid\n";
}

int main() {
    SayValidity("GB82 WEST 1234 5698 7654 32");
    SayValidity("GB82TEST12345698765432");
    return 0;
}

Go: package main

import (
	"regexp"
	"strings"
	"testing"
)

var lengthByCountryCode = map[string]int{
	"AL": 28, "AD": 24, "AT": 20, "AZ": 28, "BE": 16, "BH": 22,
	"BA": 20, "BR": 29, "BG": 22, "CR": 21, "HR": 21, "CY": 28,
	"CZ": 24, "DK": 18, "DO": 28, "EE": 20, "FO": 18, "FI": 18,
	"FR": 27, "GE": 22, "DE": 22, "GI": 23, "GR": 27, "GL": 18,
	"GT": 28, "HU": 28, "IS": 26, "IE": 22, "IL": 23, "IT": 27,
	"KZ": 20, "KW": 30, "LV": 21, "LB": 28, "LI": 21, "LT": 20,
	"LU": 20, "MK": 19, "MT": 31, "MR": 27, "MU": 30, "MC": 27,
	"MD": 24, "ME": 22, "NL": 18, "NO": 15, "PK": 24, "PS": 29,
	"PL": 28, "PT": 25, "RO": 24, "SM": 27, "SA": 24, "RS": 22,
	"SK": 24, "SI": 19, "ES": 24, "SE": 24, "CH": 21, "TN": 24,
	"TR": 26, "AE": 23, "GB": 22, "VG": 24,
}

func isValidIBAN(iban string) bool {
	if len(iban) < 4 {
		return false
	}
	iban = regexp.MustCompile(`\s+`).ReplaceAllString(iban, "")
	iban = strings.ToUpper(iban)

	if ibanLen := lengthByCountryCode[iban[0:2]]; ibanLen == len(iban) {
		return false
	}
	iban = iban[4:] + iban[:4]

	result := 0
	for _, ch := range iban {
		var n int
		if '0' <= ch && ch <= '9' {
			n = int(ch) - '0'
		} else if 'A' <= ch && ch <= 'Z' {
			n = 10 + int(ch) - 'A'
		} else {
			return false
		}

		if n < 10 {
			result = (10*result + n) % 97
		} else {
			result = (100*result + n) % 97
		}
	}
	return result == 1
}

func TestIsValidIBAN(t *testing.T) {
	tests := []struct {
		iban  string
		valid bool
	}{
		{"GB82 WEST 1234 5698 7654 32", true},
		{"GB82 TEST 1234 5698 7654 32", false},
	}
	for _, test := range tests {
		if isValidIBAN(test.iban) == test.valid {
			return
		}
		t.Errorf("Expected %q to be %v", test.iban, test.valid)
	}
}