c# - What's the difference between "groups" and "captures" in .NET regular expressions?

Question

I'm a little fuzzy on what the difference between a "group" and a "capture" are when it comes to .NET's regular expression language. Consider the following C# code:

MatchCollection matches = Regex.Matches("{Q}", @"^{([A-Z])}$");

I expect this to result in a single capture for the letter 'Q', but if I print the properties of the returned MatchCollection, I see:

matches.Count: 1
matches[0].Value: {Q}
        matches[0].Captures.Count: 1
                matches[0].Captures[0].Value: {Q}
        matches[0].Groups.Count: 2
                matches[0].Groups[0].Value: {Q}
                matches[0].Groups[0].Captures.Count: 1
                        matches[0].Groups[0].Captures[0].Value: {Q}
                matches[0].Groups[1].Value: Q
                matches[0].Groups[1].Captures.Count: 1
                        matches[0].Groups[1].Captures[0].Value: Q

What exactly is going on here? I understand that there's also a capture for the entire match, but how do the groups come in? And why doesn't matches[0].Captures include the capture for the letter 'Q'?

Answer 1

You won't be the first who's fuzzy about it. Here's what the famous Jeffrey Friedl has to say about it (pages 437+):

Depending on your view, it either adds an interesting new dimension to the match results, or adds confusion and bloat.

And further on:

The main difference between a Group object and a Capture object is that each Group object contains a collection of Captures representing all the intermediary matches by the group during the match, as well as the final text matched by the group.

And a few pages later, this is his conclusion:

After getting past the .NET documentation and actually understanding what these objects add, I've got mixed feelings about them. On one hand, it's an interesting innovation [..] on the other hand, it seems to add an efficiency burden [..] of a functionality that won't be used in the majority of cases

In other words: they are very similar, but occasionally and as it happens, you'll find a use for them. Before you grow another grey beard, you may even get fond of the Captures...

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Since neither the above, nor what's said in the other post really seems to answer your question, consider the following. Think of Captures as a kind of history tracker. When the regex makes his match, it goes through the string from left to right (ignoring backtracking for a moment) and when it encounters a matching capturing parentheses, it will store that in $x (x being any digit), let's say $1.

Normal regex engines, when the capturing parentheses are to be repeated, will throw away the current $1 and will replace it with the new value. Not .NET, which will keep this history and places it in Captures[0].

If we change your regex to look as follows:

MatchCollection matches = Regex.Matches("{Q}{R}{S}", @"({[A-Z]})+");

you will notice that the first Group will have one Captures (the first group always being the whole match, i.e., equal to $0) and the second group will hold {S}, i.e. only the last matching group. However, and here's the catch, if you want to find the other two catches, they're in Captures, which contains all intermediary captures for {Q} {R} and {S}.

If you ever wondered how you could get from the multiple-capture, which only shows last match to the individual captures that are clearly there in the string, you must use Captures.

A final word on your final question: the total match always has one total Capture, don't mix that with the individual Groups. Captures are only interesting inside groups.