Question

I am making a small applicaiton using .NET Regex types. And the “Capture, Group and Match” types totally confused me. I have never seen such an ugly solution. Could someone explain their usage for me? Many thanks.

Answer 1

Here’s a simpler example than the one in the document @Dav cited:

string s0 = @"foo%123%456%789";
Regex r0 = new Regex(@"^([a-z]+)(?:%([0-9]+))+$");
Match m0 = r0.Match(s0);
if (m0.Success)
{
  Console.WriteLine(@"full match: {0}", m0.Value);
  Console.WriteLine(@"group #1: {0}", m0.Groups[1].Value);
  Console.WriteLine(@"group #2: {0}", m0.Groups[2].Value);
  Console.WriteLine(@"group #2 captures: {0}, {1}, {2}",
                    m0.Groups[2].Captures[0].Value,
                    m0.Groups[2].Captures[1].Value,
                    m0.Groups[2].Captures[2].Value);
}

result:

full match: foo%123%456%789
group #1: foo
group #2: 789
group #2 captures: 123, 456, 789

The full match and group #1 results are straightforward, but the others require some explanation. Group #2, as you can see, is inside a non-capturing group that’s controlled by a + quantifier. It matches three times, but if you request its Value, you only get what it matched the third time around–the final capture. Similarly, if you use the $2 placeholder in a replacement string, the final capture is what gets inserted in its place.

In most regex flavors, that’s all you can get; each intermediate capture is overwritten by the next and lost; .NET is almost unique in preserving all of the captures and making them available after the match is performed. You can access them directly as I did here, or iterate through the CaptureCollection as you would a MatchCollection. There’s no equivalent for the $1-style replacement-string placeholders, though.

So the reason the API design is so ugly (as you put it) is twofold: first it was adapted from Perl’s integral regex support to .NET’s object-oriented framework; then the CaptureCollection structure was grafted onto it. Perl 6 offers a much cleaner solution, but the authors accomplished that by rewriting Perl practically from scratch and throwing backward compatibility out the window.