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