[1337Coder]

If anyone is curious about the luhn algorithm, here is one I made in C# from the example in the question:

static bool IsValidLuhn(string numbers) { if (numbers == null) throw new ArgumentNullException("number", "number must have a value.");

var allNumbers = numbers .Where((c) => c >= '0' && c <= '9') .Reverse() .Select((c, i) => (i % 2 == 1) ? ((Convert.ToInt32(c) - 48) * 2).ToString() : c.ToString());

return allNumbers.Count() > 0 ? allNumbers.Aggregate((x, y) => x + y).Sum((c) => Convert.ToInt32(c) - 48) % 10 == 0 : false; }

Edit: Can sum one link me to HackerNews markdown?

[bjconlan]

I also did one in javascript (i've requested it be ammended to the accepted response but we'll see how it reviews) anyway here it is:

  function validateCC(ccNumber) {
    var ccNumber = ccNumber.replace(/ /g, '');

    console.log(
      /^3[4|7]\d{13}/.test(ccNumber) ? 'AMEX' :
      /^6011\d{12}/.test(ccNumber) ? 'Discover' :
      /^5[1-5]\d{14}/.test(ccNumber) ? 'MasterCard' :
      /^4[\d{12}|\d{15}]/.test(ccNumber) ? 'Visa' : 'Unknown',
      ccNumber,
      ccNumber.split('').reverse()
        .map(function (v, i) { return v * (1 + i % 2) })
        .reduce(function (agg, v) { return agg + v; }, '').split('')
        .reduce(function (agg, v) { return agg + +v; }, 0) % 10 === 0 ? '(valid)' : '(invalid)');
  }

btw to markup simply append 2 spaces to the front of the newline

[ycombobreaker]

While your code may be fine, I find it weird to see functional operations in the same expression as the ASCII code '0' (48). It is a very weird juxtaposition of abstractions.