using System;

using System.Linq;

using System.Collections.Generic;       

​

public class Program

{

    public static void Main()

    {

        var ranges = "11220, 11221; 6337461000000000, 6337468549999999; 945, 965;";

        var numRanges = GetRanges(ranges);

        var testNumbers = new long[]{ 1234561234560000L, 1000000000000000L, 9500000000000000L, 6337461000000123};

        foreach(var cardNo in testNumbers)

            Console.WriteLine( "'{0}' is {1}valid!",cardNo,  IsValid(cardNo, numRanges)?"":"not " );

    }

    public static bool IsValid(long cardNumber, IEnumerable<CardRange> ranges) => ranges.Any(x => x.Contains(cardNumber));

    public static IEnumerable<CardRange> GetRanges(ReadOnlySpan<char> input)

    {

        var ranges = new List<CardRange>();

        var semiIndex = input.IndexOf(';');

        while(semiIndex > 0)

        {

            var strRange = input[..semiIndex];

            ranges.Add(CardRange.FromSpan(strRange));

            if(semiIndex < input.Length)

            {

                input = input[(semiIndex+1)..];

                semiIndex = input.IndexOf(';');

            }

            else

            {

                semiIndex = -1;

            }

        }

        return ranges;

    }

}

​

public class CardRange

{

    public long LowerBound {get; set;} = 0;

    public long UpperBound {get; set;} = long.MaxValue;

    public bool Contains(long cardNumber) => cardNumber >= LowerBound && cardNumber <= UpperBound;

    public static CardRange FromSpan(ReadOnlySpan<char> input)

    {

        var commaIndex = input.IndexOf(',');

        return new CardRange

        {

            LowerBound = long.Parse(input[..commaIndex].Trim().ToString().PadRight(16, '0')),

            UpperBound = long.Parse(input[(commaIndex+1)..].Trim().ToString().PadRight(16, '0')),

        };

    }

}