regression testing when code example

Example 1: what is regression testing

Regression testing is testing existing 
software applications to make sure that
a change or addition hasn’t broken any 
existing functionality. Its purpose is 
to catch bugs that may have been accidentally
introduced into a new build or release candidate, 
and to ensure that previously eradicated 
bugs continue to stay dead.

Example 2: regression test with example

Main idea: to make sure new code is 
working well with the old once 
Take an example of a project having multiple modules 
like Admin module, nurse module, personal details modules etc. 
there is some error in admin module like users are not able to login 
to their account even with the right credentials. 
So, this is an error and need to be fixed by the development team. 
Here, Development team modified existing code or added new code to fix 
this issue and the module is sent back to the testing team again.
Now the testing team will check either modified code has not affected 
the behavior of the rest of the application or still needs changes. 
This is called the Regression testing where new code should never 
affect the functionality of application during bug fixing.