Sample

internal class Program
{
    private static void Main(string[] args)
    {
        var dc = new DomainConfiguration("sqlserver", "Data Source=.; Initial Catalog=DO40-Tests; Integrated Security=True;");

        dc.Types.Register(typeof(TestEntity));

        dc.UpgradeMode = DomainUpgradeMode.Recreate;

        using (var d = Domain.Build(dc))
        {
            using (var s = d.OpenSession())
            using (s.Activate())
            using (var t = s.OpenTransaction())
            {
                new TestEntity(s) { String = "[test]"};
                new TestEntity(s) { String = "test"};

                t.Complete();
            }

            using (var s= d.OpenSession())
            using (s.Activate())
            using (var t = s.OpenTransaction())
            {
                Assert.AreEqual(1, Query.All<TestEntity>().Count(e => e.String.Contains("[test]")));

                t.Complete();
            }
        }
    }
}

[HierarchyRoot]
public class TestEntity : Entity
{
    /// <summary>Initializes a new instance of this class.</summary>
    /// <param name="session">The session.</param>
    public TestEntity(Session session) : base(session)
    {
    }

    [Key]
    [Field(Nullable = false)]
    public int Id { get; private set; }

    [Field]
    public string String { get; set; }
}

SQL query

SELECT COUNT_BIG(*) AS [c01umn] FROM [dbo].[TestEntity] [a] 
WHERE ([a].[String] LIKE N'%[test]%');

I assume, we need something like this

SELECT COUNT_BIG(*) AS [c01umn] FROM [dbo].[TestEntity] [a] 
WHERE ([a].[String] LIKE N'%^[test]%' ESCAPE N'^');