using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        var RemoveWords = new[] { "ABC", "DEF" };

        var testStrings = new[] {

            "123 ABC SomeWord A12",

            "123 ABCDEF",

            "ABCDEF 123",

            "X X",

            "ABC XYZ ABC DEF A1B2 ABC",

            "123 ABC 456",

            "X",

            "ABC DEF 123 456",

        };

        var removeRegex = new Regex(@"\b(?:" + string.Join("|", RemoveWords) + @")\b", RegexOptions.IgnoreCase);

        var alpha = @"\b[a-z]+\b";

        var alnum = @"\b[a-z0-9]+\b";

        var matchRegex = new Regex(string.Format(@"{0}.*{1}|{1}.*{0}", alpha, alnum), RegexOptions.IgnoreCase);

        foreach (var s in testStrings)

        {

            var ok = matchRegex.Match(removeRegex.Replace(s, "")).Success;

            Console.WriteLine("{0}:\t{1}", ok ? "OK" : "Failed", s);

        }

    }

}