Quality is delighting customers
First of all do you understand what is regression testing ?
Regression testing means if developer resolve or fix any bug or something changes are happened in already created and tested module or functionality ,testers need to verify that those resolve bugs or changes do not effect others already working modules.This testing is called regression testing.
second important thing is that don't combine retest and regression test.Both are different from the each other.
Let's take one example to understand your question.
Suppose i have three module A,B,C .Now i write 30 test cases for each module .So i have total 90 test cases.
Cycle 1 : I execute 90 test cases.From them suppose 85 passes and 5 fail.so suppose we will add 5 different defect for those 5 test cases.
Cycle 2 : Let's say that 5 defects are now fixed .so we have to identify area which are effected by fixing those defect.This is done by our experience and knowledge of system flow.suppose you found 20 test cases .So now i will execute total test cases are that 20 + 5 which were failed in 1 cycle.
suppose in 2 cycle from that 25 test cases again 1 is failed.
So this way we can identify test cases for regression test cases.
Agree with Kosha
Gud Explanation ganesh
Nice explanation given.To perform the regression testing you have to run both the new and old automated tests over the new build considering that the new test cases don't affect the working modules.