How to find the same sub-string in two strings?

ABCDEFG isn't single answer. Substrings "1", "2", "3", "12" also can be accepted as answer. You must formulate question more clearly (for example how to find same substring which length more than <n> symbols.

We *cannot* see the answer is "ABCDEFG". "ABCDEFG" may be the longest common substring, but there are more. E. g. "23" is also a common substring. "1" is also a common substring.
So please be more specific in your question.

May be I can say I want fine the longest common substring.
I want write a program to compare two file that two person keyin in chinese.
so I need a program, it's easy to find which one keyin error. just like the DOS FC.EXE do , but fc only compare the line , not every word. ex.
This is a book and that is a pen. (string a)
This are a book and I has a pen. (string b)
This <is/are> a book and <that is/I has> a pen. (The result)
I hope you can know my mean. Thanks !

I'm sorry that I cannot show my question clearly,
but this answey for search the longest common substring
is very good!

so I change to another question (I only 104 point now)
"Find the next common substring"
I hope your answer.