using System;

    using System.Collections.Generic;

    using System.Linq;                  

​

    public class Program

    {

        public static void Main()

        {

            var linesA = new string[] { "1. Dog", "2. Cat", "3. Panda" };

            var linesB = new string[] { "cat", "panda" };

            var onlyB = linesB.Intersect(linesB, new MyLineCompare());

            foreach (var b in onlyB)

                System.Console.WriteLine(b);

        }

        public class MyLineCompare : IEqualityComparer<string>

        {

            public bool Equals(string x, string y)

            {

                return (x ?? "").Equals(y, StringComparison.OrdinalIgnoreCase);

            }

            public int GetHashCode(string x)

            {

                return x.GetHashCode();

            }

        }

    }