I want to write a code that checks if it matches to a regular expression then prints matched string.

I found other question Can I use regex expression in c# with switch case?, but IsMatch method only returns boolean so I couldn’t retrieve named group string.

var filepath = @"path/to/file.txt";
foreach (string line File.ReadLines(filepath))
{
    switch (line)
    {
        case var s when new Regex(@"^(?<UserName>\w+) moved to (?<Position>\w+)$").IsMatch(s):
            // I have to know <UserName> and <Position> here.
            break;
        ...
    }
}

>Solution :

First, I’d suggest moving the regex construction itself outside the switch block – both for readability and to allow it to be reused.

Then, instead of using IsMatch, use the Match to return a Match, which you’d then check for success (potentially using a C# pattern match). For example:

using System.Text.RegularExpressions;

class Program
{
    private static void Main()
    {
        RunTest("Jon moved to XYZ");
        RunTest("Not a match");
    }

    private static readonly Regex UserMovementRegex = new Regex(@"^(?<UserName>\w+) moved to (?<Position>\w+)$");
    private static void RunTest(string line)
    {
        switch (line)
        {
            case string _ when UserMovementRegex.Match(line) is { Success: true } match:
                Console.WriteLine($"UserName: {match.Groups["UserName"]}");
                Console.WriteLine($"Position: {match.Groups["Position"]}");
                break;
            default:
                Console.WriteLine("No match");
                break;
        }
    }
}

The { Success: true } match part has two purposes:

• It checks that the match is successful
• It captures the result in a new match variable that you can use in the body of the case label