dev note (for me)

• Create 32 classes: Source, LeftString, RightString.
  • Source will be a primitive string. If I want to add property or method, I will overload Execute method which is described later.
• LeftString and RightString will inherit same interface.
• LeftString and RightString will have SearchDirection property. (I have to check dictionary later whether it is a correct English. Some websites written in English uses such expression.)
• StringExtractor will be a static class which have a Execute method. Parameters are Source, FirstString, SecondString. (I have to reconsider the naming of them. How about FirstSearch and SecondSearch?)
• Execute method should return some class instead of simple string. For future extend.
• Q.Is it possible to use callback to find index?
  A.It is difficult. For example, if you create a callback which wraps string.IndexOf method, you must have a good idea to assign the startIndex dynamically.