[c#] Regex to get NUMBER only from String

I recieve "7+" or "5+" or "+5" from XML and wants to extract only the number from string using Regex. e.g Regex.Match() function

        stringThatHaveCharacters = stringThatHaveCharacters.Trim();
        Match m = Regex.Match(stringThatHaveCharacters, "WHAT I USE HERE");
        int number = Convert.ToInt32(m.Value);
        return number;

The answers above are great. If you are in need of parsing all numbers out of a string that are nonconsecutive then the following may be of some help:

string input = "1-205-330-2342";
string result = Regex.Replace(input, @"[^\d]", "");
Console.WriteLine(result); // >> 12053302342

Either [0-9] or \d¹ should suffice if you only need a single digit. Append + if you need more.

1 The semantics are slightly different as \d potentially matches any decimal digit in any script out there that uses decimal digits.