Swedish Orgnummer Validator C#

ValidateOrgNumber.cs

using System.Linq;
using System.Text.RegularExpressions;

public bool ValidateOrgNumber(string value)
{
    // Check if the input has the correct format (6 digits followed by optional '-' and 4 more digits)
    if (!Regex.IsMatch(value, @"^\d{6}-?\d{4}$")) return false;

    // Remove any non-digit characters (e.g., '-')
    value = Regex.Replace(value, @"\D", "");

    // Reverse the string and create pairs of digits and their positions (odd/even)
    var digitPositionPairs = value.Reverse()
        .Select((c, index) => (digit: c - '0', isEvenPosition: index % 2 == 1));

    // Apply the Luhn algorithm transformation to each digit based on its position
    var transformedDigits = digitPositionPairs
        .Select(pair => pair.isEvenPosition ? pair.digit * 2 : pair.digit);

    // Subtract 9 from digits greater than 9
    var adjustedDigits = transformedDigits
        .Select(digit => digit > 9 ? digit - 9 : digit);

    // Calculate the checkSum using the adjusted digits
    int checkSum = adjustedDigits.Sum();

    // Return true if the checkSum is a multiple of 10
    return checkSum % 10 == 0;
}