using System;

using System.Text.RegularExpressions;

​

public class Program

{

    public static void Main()

    {

        var reg = new Regex(@"^A(?<adults>\d+)(\+C(?<children>\d+))?(\+I(?<infants>\d+))?$", RegexOptions.Compiled);

​

        var tests = new String[] { "A3", "A3+C4", "A22+C4+I9" };

        for (var i = 0; i < tests.Length; i++)

        {

            var tested = tests[i];

            Console.WriteLine($"Tested string: {tested}");

            var matches = reg.Match(tested);

            Console.WriteLine($"Adults: {matches.Groups["adults"]}");

            Console.WriteLine($"Children: {matches.Groups["children"]}");

            Console.WriteLine($"Infants: {matches.Groups["infants"]}\n");

        }

    }

}