Task: IBAN
C: #include <alloca.h>
#include <ctype.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define V(cc, exp) if (!strncmp(iban, cc, 2)) return len == exp


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

    return 0;
}


int strip(char *s)
{
    int i = -1, m = 0;

    while(s[++i]) {
        s[i - m] = s[i];
        m += s[i] <= 32;
    }

    s[i - m] = 0;
    return i - m;
}


int mod97(const char *s, int len)
{
    int i, j, parts = len / 7;
    char rem[10] = "00";

    for (i = 1; i <= parts + (len % 7 != 0); ++i) {
        strncpy(rem + 2, s + (i - 1) * 7, 7);
        j = atoi(rem) % 97;
        rem[0] = j / 10 + '0';
        rem[1] = j % 10 + '0';
    }

    return atoi(rem) % 97;
}

int valid_iban(char *iban)
{
    int i, j, l = 0, sz = strip(iban);
    char *rot, *trans;

    
    for (i = 0; i < sz; ++i) {
        if (!isdigit(iban[i]) && !isupper(iban[i]))
            return 0;
        l += !!isupper(iban[i]);
    }

    if (!valid_cc(iban, sz))
        return 0;

    
    rot = alloca(sz);
    strcpy(rot, iban + 4);
    strncpy(rot + sz - 4, iban, 4);

    
    trans = alloca(sz + l + 1);
    trans[sz + l] = 0;

    
    for (i = j = 0; i < sz; ++i, ++j) {
        if (isdigit(rot[i]))
            trans[j] = rot[i];
        else {
            trans[j]   = (rot[i] - 55) / 10 + '0';
            trans[++j] = (rot[i] - 55) % 10 + '0';
        }
    }

    return mod97(trans, sz + l) == 1;
}

int main(int _, char **argv)
{
    while (--_, *++argv)
        printf("%s is %svalid.\n", *argv, valid_iban(*argv) ? "" : "in");

    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)
	}
}