How do I validate an Australian Medicare number?

Here's a Typescript or modern Javascript solution:

  validateMedicare(medicare) {
    let isValid = false;

    if (medicare && medicare.length === 10) {
      const matches = medicare.match(/^(\d{8})(\d)/);

      if (!matches) {
        return { invalid: true };
      }

      const base = matches[1];
      const checkDigit = matches[2];
      const weights = [1, 3, 7, 9, 1, 3, 7, 9];

      let sum = 0;
      for (let i = 0; i < weights.length; i++) {
        sum += parseInt(base[i], 10) * weights[i];
      }

      isValid = sum % 10 === parseInt(checkDigit, 10);
    }

    return isValid;
  }

Please refer to http://clearwater.com.au/code/medicare for an explanation.

To test, generate medicare number here: https://precedencehealthcare.com/rmig/

Added Dart version:

bool isMedicareValid(String input, {bool validateWithIrn = true}) {
  final medicareNumber = input.replaceAll(" ", "");
  final length = validateWithIrn ? 11 : 10;

  if (medicareNumber.length != length) {
    return false;
  }

  final multipliers = [1, 3, 7, 9, 1, 3, 7, 9];
  final regexp = RegExp("^(\\d{8})(\\d)", caseSensitive: false);

  final matches = regexp.allMatches(medicareNumber).toList();

  if (matches.length > 0 && matches[0].groupCount >= 2) {
    final base = matches[0].group(1);
    final checkDigit = matches[0].group(2);
    var total = Iterable.generate(multipliers.length)
        .fold(0, (current, index) => current += (int.tryParse(base[index]) * multipliers[index]));

    return (total % 10) == int.parse(checkDigit);
  }
  return false;
}

Another Swift implementation with comments:

func validateMedicareNumber(input: String) -> Bool {
    let weights = [1, 3, 7, 9, 1, 3, 7, 9]

    // Remove all whitespace
    var trimmed = input.trimmingCharacters(in: .whitespacesAndNewlines)
        .components(separatedBy: .whitespaces)
        .joined()

    // The medicare card number has 8 digits identifier + 1 digit checksum + 1 digit issue number.
    // Either 9 or 10 numbers will be valid
    guard trimmed.count == 9 || trimmed.count == 10 else {
        return false
    }

    // Drop the issue number if it was added
    if trimmed.count == 10 {
        trimmed = String(trimmed.dropLast())
    }

    // The last digit is a checksum
    guard let lastElement = trimmed.last,
        let checkSum = Int(String(lastElement)) else {
        return false
    }

    // Remove the checksum from our input
    trimmed = String(trimmed.dropLast())

    // Multiply the numbers by weights
    let weightedNumbers: [Int] = trimmed.enumerated().map { index, element in
        guard let number = Int(String(element)) else {
            // -1 will throw the calculations out of the window which
            // will guarantee invalid checksum
            return -1
        }

        return number * weights[index]
    }

    // Validate the weighted numbers against the checksum
    return weightedNumbers.reduce(0, +) % 10 == checkSum
}

If you need a test card number for development, use this one. It's for John Doe

TEST CARD NUMBER > 2428 77813 2/1

Expanding on Daniel Ormeño answer, for asp.net you can put the check into an attribute and decorate the property in the model

public class MedicareValidation : ValidationAttribute
{
    public override bool IsValid(object value)
    {
        string medicareNumber = value.ToString();

        if (!(medicareNumber?.Length >= 10 && medicareNumber.Length < 12) || !medicareNumber.All(char.IsDigit))
            return false;

        var digits = medicareNumber.Select(c => (int)char.GetNumericValue(c)).ToArray();
        return digits[8] == GetMedicareChecksum(digits.Take(8).ToArray());
    }

    private int GetMedicareChecksum(int[] digits)
    {
        return digits.Zip(new[] { 1, 3, 7, 9, 1, 3, 7, 9 }, (m, d) => m * d).Sum() % 10;
    }
}

Then in the model

[DisplayName("Medicare Number")]  
[MedicareValidation] 
public string MedicareNumber {get; set;}

Here's a Python version! Note that it expects a full 11 digit Medicare number. If you're validating 10 digit Medicare numbers without the individual reference number, you'll need to tweak the regex in the re.match line.

def validate_medicare_number(medicare_number: str) -> bool:
    """Given a string containing a medicare number, return True if valid,
    False if invalid.

    >>> validate_medicare_number("2428 77813 2/1")
    True
    >>> validate_medicare_number("7428 77818 2/1")
    False
    >>> validate_medicare_number("2428 77815 2/1")
    False
    """

    # See here for checksum algorithm:
    # https://stackoverflow.com/a/15823818
    # https://clearwater.com.au/code/medicare

    # Remove whitespace
    medicare_number = re.sub(r"[^\d]+", "", medicare_number)
    if re.match(r"^[2-6]\d{10}$", medicare_number):

        medicare_digits = list(map(int, medicare_number))
        checksum_weights = (1, 3, 7, 9) * 2
        digit_weight_pairs = zip(medicare_digits, checksum_weights)
        checksum = sum([digit * weight for digit, weight in digit_weight_pairs]) % 10

        return checksum == medicare_digits[8]
    else:
        return False

You can use simple regex validation: .replace(/\W/gi, "") .replace(/(.{4})(.{5})/g, "$1 $2 ");

check my example here: codesandbox.io