C# – What’s the difference between “groups” and “captures” in .NET regular expressions

cnetregex

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'?

Best Answer

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...

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.

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