How are non-capturing groups, i.e., (?:)
, used in regular expressions and what are they good for?
Regex Non-Capturing Group – What is a Non-Capturing Group in Regular Expressions?
capturing-groupregexregex-group
Related Solutions
It means that the grouping is atomic, and it throws away backtracking information for a matched group. So, this expression is possessive; it won't back off even if doing so is the only way for the regex as a whole to succeed. It's "independent" in the sense that it doesn't cooperate, via backtracking, with other elements of the regex to ensure a match.
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.
Best Answer
Let me try to explain this with an example.
Consider the following text:
Now, if I apply the regex below over it (I did not escape the slashes for clarity; when using it, slashes would have to be escaped to
\/
)...... I would get the following result:
But I don't care about the protocol -- I just want the host and path of the URL. So, I change the regex to include the non-capturing group
(?:)
.Now, my result looks like this:
See? The first group has not been captured. The parser uses it to match the text, but ignores it later, in the final result.
EDIT:
As requested, let me try to explain groups too.
Well, groups serve many purposes. They can help you to extract exact information from a bigger match (which can also be named), they let you rematch a previous matched group, and can be used for substitutions. Let's try some examples, shall we?
Imagine you have some kind of XML or HTML (be aware that regex may not be the best tool for the job, but it is nice as an example). You want to parse the tags, so you could do something like this (I have added spaces to make it easier to understand):
The first regex has a named group (TAG), while the second one uses a common group. Both regexes do the same thing: they use the value from the first group (the name of the tag) to match the closing tag. The difference is that the first one uses the name to match the value, and the second one uses the group index (which starts at 1).
Let's try some substitutions now. Consider the following text:
Now, let's use this dumb regex over it:
This regex matches words with at least 3 characters, and uses groups to separate the first three letters. The result is this:
So, if we apply the substitution string:
... over it, we are trying to use the first group, add an underscore, use the third group, then the second group, add another underscore, and then the fourth group. The resulting string would be like the one below.
You can use named groups for substitutions too, using
${name}
.To play around with regexes, I recommend http://regex101.com/, which offers a good amount of details on how the regex works; it also offers a few regex engines to choose from.