Question

What am I doing wrong? I have the () to set up two Groups, and I expect sExtractNumber to create Groups with values of “0.234” and “”, but they both have .Value = “0.234”?

const string REGEX_NUMBER = "[0-9\\.-]*";
static readonly Regex sExtractNumber = new 
     Regex(string.Format("^({0})(.*)$", REGEX_NUMBER),
     RegexOptions.Singleline | RegexOptions.Compiled);

–

[Test]  public void ParseNumber()
{
  double num;
  string rest;
  Assert.True(KbParser.ExtractNumber("0.234", out num, out rest));
  Assert.AreEqual(0.234, num, 0.0001);
  Assert.AreEqual(rest, "in");  // fails .  Rest == "0.234"

  Assert.True(KbParser.ExtractNumber("0.234in", out num, out rest));
  Assert.AreEqual(0.234, num, 0.0001);
  Assert.AreEqual(rest, "in");
}

–

public static bool ExtractNumber(string name, out double number, out string rest)
{
  Match m = sExtractNumber.Match(name);
  string numbertext = m.Groups[0].Value;
  rest = m.Groups[1].Value;
  return double.TryParse(numbertext, out number);
}

Answer 1

Groups[0] always contains the text matched. The first captured group is in Groups[1]. Therefore, your ExtractNumber method should be:

public static bool ExtractNumber(string name, out double number, out string rest)
{
  Match m = sExtractNumber.Match(name);
  string numbertext = m.Groups[1].Value;
  rest = m.Groups[2].Value;
  return double.TryParse(numbertext, out number);
}