public class IsMatchTests : Linq3IntegrationTest
{
    [Fact]
    public void Regex_IsMatch()
    {
        var collection = CreateCollection();

        var query = collection.AsQueryable()
            .Where(i => Regex.IsMatch((string)i.A, @"\dB.*", RegexOptions.ECMAScript));

        var results = query.ToList();

        results.Should().OnlyContain(i => Regex.IsMatch(i.A, @"\dB.*", RegexOptions.ECMAScript));
    }

    private IMongoCollection<Data> CreateCollection()
    {
        var collection = GetCollection<Data>("test");
        CreateCollection(
            collection,
            new Data { Id = 1, A = "ABC" },
            new Data { Id = 2, A = "1Br" });
        return collection;
    }

    private class Data
    {
        public int Id { get; set; }
        public string A { get; set; }
    }
}