String cannot be of zero length. Parameter name: oldValue

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Text.RegularExpressions;

public partial class Add_Details : System.Web.UI.Page
{
    private string Searchstring = "";
    protected void Page_Load(object sender, EventArgs e)
    {

    }
    public string HighlightText(string InputTxt)
    {
        string Search_Str = txtSearch.Text;
        Regex RegExp =new Regex(Search_Str.Replace("","|").Trim(), RegexOptions.IgnoreCase);
        return RegExp.Replace(InputTxt, new MatchEvaluator(Replacekeywords));

    }
    public string Replacekeywords(Match m)
    {
        return ("<span class=highlight>" + m.Value + "</span>");
    }
    protected void btnSearch_Click(object sender, EventArgs e)
    {
        Searchstring = txtSearch.Text;

    }
    protected void clear_Click(object sender, EventArgs e)
    {
        txtSearch.Text = "";
        Searchstring = "";
        gvDetails.DataBind();
    }
}