Regex match.name returns 0 not the name of the capturing group

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Hi I have this

// ... Input string.
            string input = "Left12345Right";

            // ... Use named group in regular expression.
            Regex expression = new Regex(@"Left(?<middle>\d+)Right");

            // ... See if we matched.
            Match match = expression.Match(input);
            if (match.Success)
            {
                // ... Get group by name.
                string result = match.Groups["middle"].Value;
                Console.WriteLine("Middle: {0}", result);
                Console.WriteLine(match.Name);
            }
            // Done.
            Console.ReadLine();

As I read the info, .Name should return the name of the capturing group, but for me it only returns 0, and I can't understand why.

So I hope that some here can tel me what I'm doing wrong. (I'm new to C# and regex).

Cheers
/LR

What I have tried:

Googled to find answer, but found nothing.

Posted 4-Feb-20 6:14am

Lupu5R3x

Updated 4-Feb-20 6:46am

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Comments

Richard MacCutchan 4-Feb-20 12:36pm

According to my tests, and the documentation, Match does not contain a Name property.

Richard Deeming 4-Feb-20 12:38pm

It inherits the Name property from the Group class:
Match Class (System.Text.RegularExpressions) | Microsoft Docs[^]
Group.Name Property (System.Text.RegularExpressions) | Microsoft Docs[^]

Richard MacCutchan 4-Feb-20 12:53pm

The compiler rejected it when I tried. I Must take a longer look at that.

2 solutions

Solution 1

Try this:

C#

Console.WriteLine(match.Groups[1].Name);

The first group in a Match is the whole match, which consists of all the Groups in the Match.

Posted 4-Feb-20 6:43am

OriginalGriff

Comments

Maciej Los 4-Feb-20 14:24pm

5ed!

Lupu5R3x 5-Feb-20 11:43am

Hi I tried that, but [1] just give me the name of the first group name in my pattern, not the name of the captured group.

eg. if I have this pattern (?(abc)|(?<d>def)|(?<g>ghi) and test string def [1] just gives me a :(

/LR

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Richard Deeming · Accepted Answer · 2020-02-04T06:46:00

Solution 2

Match Class (System.Text.RegularExpressions) | Microsoft Docs[^]:
The Match instance itself is equivalent to the first object in the collection, at Match.Groups[0], which represents the entire match.

There is no sensible name that could be given to the group representing the entire match, so it gets the name 0 by default. This allows you to reference the entire match in a replacement expression by using $0.

It's not clear from your question what you're trying to achieve with the Name property. Were you looking for the GetGroupNames method[^] instead?

Posted 4-Feb-20 6:46am

Richard Deeming

Comments

Maciej Los 4-Feb-20 14:24pm

5ed!

Lupu5R3x 5-Feb-20 11:23am

I'm trying to convert a Autoit regex pattern like this "(abc)|(def)|(ghi)", 1
Where ,1 returns a match of arrays.

So the result of "def" would return [0] = , [1] = def

But sins I couldn't find any equitant to match of arrays in C#, my hope was that I could use naming groups to return the name of the group that was matched.

eg. (?abc)|(?<d>def)|(?<g>ghi) and test def should (was my thought) return d.

But when trying what @OriginalGriff sugested, I just get the group name I specified so [1] returns a, [2] = b but that it does regardless of what the match is.

And yes I should have specified exactly what I was trying to achieve, with the regex, sry for that :(

/LR

Richard Deeming 5-Feb-20 11:46am

I'm not familiar with the "match of arrays" feature, but named groups seems like the way to go:

Regex expression = new Regex(@"(?<a>abc)|(?<b>def)|(?<c>ghi)", RegexOptions.ExplicitCapture);
Match match = expression.Match(input);
if (match.Success)
{
    if (match.Groups["a"].Success)
    {
        // Matched first option...
    }
    else if (match.Groups["b"].Success)
    {
        // Matched second option...
    }
    else if (match.Groups["c"].Success)
    {
        // Matched third option...
    }
}

Lupu5R3x 5-Feb-20 12:17pm

Hmm, yes it seems that it would be the most simple solution to my problem. :)

Regardless of how annoying it is that all my one line regex's end up as several lines in C# to get the same result :( the last regex I translated went from one line to 10 lines in C# :O course C# don't support \K :'(

/LR

Richard Deeming 5-Feb-20 12:34pm

I'm not familiar with \K - is it some sort of look-behind operator?

Grouping Constructs in Regular Expressions | Microsoft Docs[^]

string input = "2010 1999 1861 2140 2009";
string pattern = @"(?<=\b20)\d{2}\b";
foreach (Match match in Regex.Matches(input, pattern))
{
    Console.WriteLine(match.Value);
}

/*
Output:
10
09
*/

Lupu5R3x 5-Feb-20 12:47pm

\K "Resets start of match at the current point in subject string. Note that groups already captured are left alone and still populate the returned array; it is therefore always possible to backreference to them later on. Action of \K is similar but not identical to a look-behind, in that \K can work on alternations of varying lengths."

So this regreplace".{1,60}\K(\s{1,2}|$)", "\r\n"
which returned one string "splitted" by 60 char in one string at once, ended up as this.
string splitPattern = @"(.{1,60})(\s{1,2}|$)";
RegexOptions regexOptions = RegexOptions.IgnoreCase;
Regex splitText = new Regex(splitPattern, regexOptions);
StringBuilder result = new StringBuilder();

foreach (Match match in splitText.Matches(input))
{
if (match.Success)
{
result.Append(match.Groups[0].Value + "\r\n");
}
}
return result.ToString();

/LR