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GetGroupNumbers Method

Regex.GetGroupNumbers Method

Returns an array of capturing group numbers that correspond to group names in an array.

Namespace:  System.Text.RegularExpressions
Assemblies:   System.Text.RegularExpressions (in System.Text.RegularExpressions.dll)
  System (in System.dll)

public int[] GetGroupNumbers()

Return Value

Type: System.Int32[]
An integer array of group numbers.

Both unnamed and named capturing groups can be accessed by number. Unnamed groups are numbered from left to right starting with 1. (The capturing group in index 0 (zero) represents the match as a whole.) Named groups are then numbered from left to right starting with a number that is one greater than the number of unnamed capturing groups.

Referencing a group by its number instead of by string name can provide faster access.

The following example defines a regular expression, \b((?<word>\w+)\s*)+(?<end>[.?!]), that matches a sentence. The regular expression includes three capturing groups: an unnamed group that captures an individual word along with a space character that may follow it; a group named word that captures the individual words in the sentence; and a group named end that captures the punctuation that ends the sentence. The example calls the GetGroupNumbers method to get the numbers of all capturing groups, and then displays their captured string. In addition, the GroupNameFromNumber method is used to indicate whether a particular numbered group corresponds to a named group.

using System;
using System.Text.RegularExpressions;

public class Example
   public static void Main()
      string pattern = @"\b((?<word>\w+)\s*)+(?<end>[.?!])";
      string input = "This is a sentence. This is a second sentence.";

      Regex rgx = new Regex(pattern);
      int[] groupNumbers = rgx.GetGroupNumbers();
      Match m = rgx.Match(input);
      if (m.Success) {
         Console.WriteLine("Match: {0}", m.Value);
         foreach (var groupNumber in groupNumbers) {
            string name = rgx.GroupNameFromNumber(groupNumber);
            int number;
            Console.WriteLine("   Group {0}{1}: '{2}'", 
                              ! string.IsNullOrEmpty(name) & 
                              ! Int32.TryParse(name, out number) ?
                                 " (" + name + ")" : String.Empty, 
// The example displays the following output: 
//       Match: This is a sentence. 
//          Group 0: 'This is a sentence.' 
//          Group 1: 'sentence' 
//          Group 2 (word): 'sentence' 
//          Group 3 (end): '.'

The regular expression pattern is interpreted as shown in the following table.




Begin the match at a word boundary.


Match one or more word characters and assign the matched strings to a group named word.


Match zero or more white-space characters.


Assign the word captured group followed by any captured white-space characters to the first captured group.


Match the pattern of one or more word characters followed by any white-space characters one or more times.


Match a period, question mark, or exclamation point. Assign the matched character to the end capturing group.

.NET Framework

Supported in: 4.6, 4.5, 4, 3.5, 3.0, 2.0, 1.1

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

XNA Framework

Supported in: 3.0, 2.0, 1.0

.NET for Windows Phone apps

Supported in: Windows Phone 8.1, Windows Phone Silverlight 8.1, Windows Phone Silverlight 8

Portable Class Library

Supported in: Portable Class Library
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