Regex: How to get a group name

Question

I have a .NET Regex which looks similar to:

(?<Type1>AAA)|(?<Type2>BBB)

I'm using the Matches method against a sample string e.g. "AAABBBAAA", and then iterating over the matches.

My goal is to find the match types, using the regex match group, so for this regex it will be:

• Type1
• Type2
• Type1

I couldn't find any GetGroupName method. Please help.

Answer 1

If you want to retrieve a specific group name you can use the Regex.GroupNameFromNumber method.

//regular expression with a named group
Regex regex = new Regex(@"(?<Type1>AAA)|(?<Type2>BBB)", RegexOptions.Compiled);

//evaluate results of Regex and for each match
foreach (Match m in regex.Matches("AAABBBAAA"))
{
    //loop through all the groups in current match
    for(int x = 1; x < m.Groups.Count; x ++)
    {
        //print the names wherever there is a succesful match
        if(m.Group[x].Success)
            Console.WriteLine(regex.GroupNameFromNumber(x));
    }
}

Also, There is a string indexer on the GroupCollection. object that is accesible on the Match.Groups property, this allows you to access a group in a match by name instead of by index.

//regular expression with a named group
Regex regex = new Regex(@"(?<Type1>AAA)|(?<Type2>BBB)", RegexOptions.Compiled);

//evaluate results of Regex and for each match
foreach (Match m in regex.Matches("AAABBBAAA"))
{
    //print the value of the named group
    if(m.Groups["Type1"].Success)
        Console.WriteLine(m.Groups["Type1"].Value);
    if(m.Groups["Type2"].Success)
        Console.WriteLine(m.Groups["Type2"].Value);
}

Answer 2

Is this the sort of thing you're looking for? It uses Regex.GroupNameFromNumber so you don't need to know the group names outside the regex itself.

using System;
using System.Text.RegularExpressions;

class Test
{
    static void Main()
    {
        Regex regex = new Regex("(?<Type1>AAA)|(?<Type2>BBB)");
        foreach (Match match in regex.Matches("AAABBBAAA"))
        {
            Console.WriteLine("Next match:");
            GroupCollection collection = match.Groups;
            // Note that group 0 is always the whole match
            for (int i = 1; i < collection.Count; i++)
            {
                Group group = collection[i];
                string name = regex.GroupNameFromNumber(i);
                Console.WriteLine("{0}: {1} {2}", name, 
                                  group.Success, group.Value);
            }
        }
    }
}

Answer 3

Try this:

        var regex = new Regex("(?<Type1>AAA)|(?<Type2>BBB)");
        var input = "AAABBBAAA";
        foreach (Match match in regex.Matches(input))
        {
            Console.Write(match.Value);
            Console.Write(": ");
            for (int i = 1; i < match.Groups.Count; i++)
            {
                var group = match.Groups[i];
                if (group.Success)
                    Console.WriteLine(regex.GroupNameFromNumber(i));
            }    
        }