Find a string between 2 known values

Solution without need of regular expression:

string ExtractString(string s, string tag) {
     // You should check for errors in real-world code, omitted for brevity
     var startTag = "<" + tag + ">";
     int startIndex = s.IndexOf(startTag) + startTag.Length;
     int endIndex = s.IndexOf("</" + tag + ">", startIndex);
     return s.Substring(startIndex, endIndex - startIndex);
}

  Regex regex = new Regex("<tag1>(.*)</tag1>");
  var v = regex.Match("morenonxmldata<tag1>0002</tag1>morenonxmldata");
  string s = v.Groups[1].ToString();

Or (as mentioned in the comments) to match the minimal subset:

  Regex regex = new Regex("<tag1>(.*?)</tag1>");

Regex class is in System.Text.RegularExpressions namespace.