using System;

using System.Text.RegularExpressions;

​

public class Program

{

    public static void Main()

    {

        var conteudo = @"........................ -Dfile.jar=C:\JAR.jar -cp .........";

        var pattern = @"-Dfile.jar=(.*)\s-cp(.)*";

        Regex r = new Regex(pattern, RegexOptions.IgnoreCase);      

        Match m = r.Match(conteudo);

        if (m.Success) 

        {

            Console.WriteLine(m.Groups[1]);

        }

    }

}