Regex capturing

Question

I am having trouble capturing a value from a string. I only want the number I don't want to capture the T or :. This failing test explains:

[TestMethod]
public void RegExTest()
{
    var rex = new Regex("^T([0-9]+):"); //as far as I understand, the () denote the capture group
    var match = rex.Match("T12:abc");
    Assert.IsTrue(match.Success);
    Assert.IsTrue(match.Groups[0].Success);
    Assert.AreEqual("12", match.Groups[0].Captures[0]); //fails, actual is "T12:"
}

Whatever you don't want to capture you must denote it. Your regex should be `^(?:T)([0-9]+)(?:\:)$` — Andre Calil, Aug 22 '12 at 14:53
@AndreCalil I see, yes that works, except I have to use `Groups[1]` to get the result. Thanks. — weston, Aug 22 '12 at 15:00

score 1 · Accepted Answer · answered Aug 22 '12 at 15:00

Groups collection, which is zero based, denotes capturing groups from index 1.
Groups[0] always indicates the entire match.
Hence you need to do Groups[1] instead of Groups[0] above.

The MatchGroups property returns a GroupCollection object that contains Group objects that represent captured groups in a single match. The first Group object in the collection (at index 0) represents the entire match. Each object that follows represents the results of a single capturing group.

The Group Collection

Anirudha · Answer 2 · 2012-08-22T15:40:32.217

1

So you want to match digits between T and :

Here is a simple Regex for that

@"(?<=T)\d+(?=:)"//no need of groups here

About your Regex:

your regex

^T([0-9]+):

should be like this

T(\d+)://^ is not needed and [0-9] can be represented as \d

Here

Group[0] would be T:12//a full match
Group[1] would be 12//1st match within ()i.e.1st ()
Group[2] would be //2nd match within ()i.e 2nd ()

edited Aug 22 '12 at 15:40

answered Aug 22 '12 at 15:09

Anirudha

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Thanks, the `^` is to ensure that the T is at the start of the string. So I do need that don't I? – weston Aug 22 '12 at 15:34
no you don't need it... if you use `^` then a string **sddsT55:** would not match since `^` means that you want T at the begging.So a string **T55:** would match with `^` but not **sddsT55:** – Anirudha Aug 22 '12 at 15:38
@weston if you are sure that the `string` would be always `T54:` then you should use `^` – Anirudha Aug 22 '12 at 15:43
Yeah I'm sure, see my next question makes it clearer http://stackoverflow.com/questions/12077136/regex-extract-optional-group – weston Aug 22 '12 at 16:07

score 0 · Answer 3 · edited May 23 '17 at 12:27

0

Got it by naming the group.

[TestMethod]
public void RegExTest()
{
    var rex = new Regex("^T(?<N>[0-9]+):");
    var match = rex.Match("T12:abc");
    Assert.IsTrue(match.Success);
    Assert.IsTrue(match.Groups[0].Success);
    Assert.AreEqual("12", match.Groups["N"].Value);
}

Should have looked harder: How do I access named capturing groups in a .NET Regex?

edited May 23 '17 at 12:27

Community

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answered Aug 22 '12 at 14:56

weston

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Well, congrats for answering your own question. Could you give a try to my suggestion, anyway? – Andre Calil Aug 22 '12 at 14:59
@AndreCalil I did, and it works, but like I said only by accessing `Group[1]`, which actually as Kash points out works without changing the expression. – weston Aug 22 '12 at 15:03

Regex capturing

3 Answers3