C#

string sFull = "this the is test for the true";
            string sToSearch = "the";
            int iFirst = sFull.IndexOf(sToSearch);
            int iSecond = sFull.IndexOf(sToSearch, iFirst + sToSearch.Length);
            string sText = sFull.Substring(iFirst + sToSearch.Length, iSecond - iFirst -sToSearch.Length);

TextBox[] myTextBoxes = new TextBox[5]

for(int index = 0; index < myTextBoxes.Length; index++)
{
myTextBoxes[index] = new TextBox();
myTextBoxes[index].Text = String.Format("TextBox #{0}", index+1);
}

foreach(Control c in this.Controls)
{
   if(c is TextBox)
      ((TextBox)c).Text = insert_your_text_here;
}