Question

How do I do this? I’ve seen a solution not using a single regex for ruby becauase ruby doesn’t support loookaround assertions. But is it possible in c#?

[Test]
public void RarArchiveFirstFileNameShouldMatch() {
    var regex = new Regex(@"\.(rar|001)$", RegexOptions.IgnoreCase | RegexOptions.Singleline);
    Assert.That(regex.IsMatch("filename.001"));
    Assert.That(regex.IsMatch("filename.rar"));
    Assert.That(regex.IsMatch("filename.part1.rar"));
    Assert.That(regex.IsMatch("filename.part01.rar"));
    Assert.That(regex.IsMatch("filenamepart44.rar"));
    Assert.That(regex.IsMatch("filename.004"), Is.False);
    Assert.That(regex.IsMatch("filename.057"), Is.False);
    Assert.That(regex.IsMatch("filename.r67"), Is.False);
    Assert.That(regex.IsMatch("filename.s89"), Is.False);
    Assert.That(regex.IsMatch("filename.part2.rar"), Is.False);
    Assert.That(regex.IsMatch("filename.part04.rar"), Is.False);
    Assert.That(regex.IsMatch("filename.part11.rar"), Is.False);
}

Answer 1

This should pass your tests:

    var regex = new Regex(@"(\.001|\.part0*1\.rar|^((?!part\d*\.rar$).)*\.rar)$", RegexOptions.IgnoreCase | RegexOptions.Singleline);
    Assert.That(regex.IsMatch("filename.001"));
    Assert.That(regex.IsMatch("filename.rar"));
    Assert.That(regex.IsMatch("filename.part1.rar"));
    Assert.That(regex.IsMatch("filename.part01.rar"));
    Assert.That(regex.IsMatch("filename.004"), Is.False);
    Assert.That(regex.IsMatch("filename.057"), Is.False);
    Assert.That(regex.IsMatch("filename.r67"), Is.False);
    Assert.That(regex.IsMatch("filename.s89"), Is.False);
    Assert.That(regex.IsMatch("filename.part2.rar"), Is.False);
    Assert.That(regex.IsMatch("filename.part04.rar"), Is.False);
    Assert.That(regex.IsMatch("filename.part11.rar"), Is.False);