using System;

using System.Text.RegularExpressions;

using System.Linq;

public class Program

{

    public static void Main()

    {

        string file = @"data:image/jpeg;base64,/9j/4AAQSkZJRgABAQEASABIAAD";

        var regex = new Regex(@"^data:(.*);base64");

        var matches = regex.Matches(file);

              // Report on each match.

        foreach (Match match in matches)

        {

            GroupCollection groups = match.Groups;

            Console.WriteLine(groups[1].Value);

        }

    }

}