What is Regression Testing?

Today’s software development is all about being fast and flexible, which lets us create very advanced and complex programs. But sometimes, even the best systems can have problems after we change the code. When this happens, we need to do a check-up called regression testing. This article will explain regression testing in an easy way, tell you why it’s needed, how to do it, how it’s different from other tests, and the best ways to make sure your software stays working well.

Regression testing is a type of testing that ensures that the existing functions in a system remain unaffected, and no decline occurs whenever there are any code changes in the system. It confirms that no new bugs are introduced as a result of code changes, and the existing system does not break in any way.

Regression testing ensures system stability, performance, usability, and reliability by re-running previously executed tests for the existing system — that’s already passed — whenever there’s a change in the software source code.

For instance, suppose you go to a radio repair shop to get your old radio’s volume knob fixed. Once the volume knob is repaired and tested, you also confirm that other functions in your radio — like the channel tuner, antenna settings, etc. — aren’t affected due to the repair work. Simply put, you regress or return to validate that other functions of your radio work, without problems.

Note that regression testing can either be a white box testing method, where you need to understand internal code working to run tests — or a black box testing method, where you don’t need to understand code.

Regression testing aims to evaluate and keep software quality top-notch — using regression test suites — after the system undergoes any modifications, additions, or changes in the source code. This helps to sniff out any bugs or issues that may have occurred as a side effect of code changes such as:

• Fixing existing bugs in the system
• Addition of new features
• Modifications of existing features
• Major or minor improvements made to the system
• New product or new version release

Also Read: What is Bug Life Cycle and How To Run it?

Here’s a graphical representation that portrays the regression testing process:

The above graph shows the regression testing of features F1, F2, and F3 when the fourth version of the software is released. Regression testing confirms a stable working V4 build when a new feature (F4) gets added to the system by subjecting all the features under test.

For instance, suppose F1, F2, and F3 initially underwent rigorous testing procedures with test scenarios, covering 100 test cases that were passed, and then 20 new test cases were defined for F4.

With regression testing, those 20 test cases would be performed for F4 along with re-running the 100 tests (that’s already passed) for the features F1, F2, and F3, to ascertain that the new feature (F4) does not affect the existing features (F1, F2, and F3) or the system functionality in any way.

Automation is the preferred way to run regression tests. Here’s why:

• Frequent test runs — regression tests are often needed during or after software updates.  Automating them helps get the job done quickly and without much hassle.
• Consistent quality — automated tests don’t make human-like errors. They’re reliable for producing consistent and accurate results, every time you run the regression tests suite.
• Saves time and money — automation facilitates efficient regression testing, especially when you can run automated at odd hours or simultaneously, cutting down on manual testing costs.
• Thorough testing — automation can handle several tricky test scenarios in regression testing, ensuring every part of the system gets checked. This significantly maximizes test coverage.
• Quick bug finds — when automated regression tests are part of the regular development cycle, especially with the CI/CD pipelines, issues are spotted earlier and bug reports come in faster for prompt fixes, avoiding bigger issues later.
• Smart use of resources — with automated regression tests, the testing team can focus more on complex issues that need a human touch.

Also Read: What is Automated Debugging?

Some prominent automation tools for regression testing include:

• Selenium
• testRigor
• Katalon Studio
• Sahi Pro
• IBM Rational Functional Tester (RFT)

Also Read: Hot Automation Testing Tools in 2024

Here’s how you can run regression testing for your software:

Step 1# Test Case Selection

Select the necessary test cases — that’s already passed — to be re-executed to verify that the existing functions work well after the current code changes.

For instance, suppose you have a social media app, where a new feature (F3) for photo filters is added. The test case selection would include the test cases for the photo filter feature (F3) and related features — that have already been validated in previous testing cycles — like image uploading (F1) and profile updates (F2) — to ensure they are not affected.

Also Read: STLC: Getting Started with Software Testing

Step 2# Time Estimation

Plan and allocate necessary time (and resources), based on requirements to minimize any unexpected delays or blockers. Regression testing is a long-running and resource-intensive process. Hence, proper time estimation is key to getting a glimpse of the upcoming procedures.

For instance — in the social media app, if the previous testing cycle for features F1 and F2 took two days to complete 100 test cases, and now there are 20 new test cases added for feature F3, the time and resource estimation helps consider the complexity of new test cases, to allocate the optimal duration to run tests for the new feature along with the old ones.

Step 3# Test Case Automation

Automate test cases, wherever possible, using automation frameworks and tools to step up efficiency and reduce the time required to run regression tests, especially for repetitive or long-running tasks.

For instance — automating the test cases for the login and logout functionality of the app, as these are repetitive and can be time-consuming; if done manually.

Step 4# Test Case Prioritization

Prioritize test cases based on importance, their impact on the existing system, and the criticality of the features.

For instance — prioritizing test cases related to the payment system of an e-commerce app, over the test cases for the review section, as the payment system is more critical to the business.

Step 5# Test Execution

Execute the tests based on selection and prioritization, to validate that the system build is stable, performant, usable, reliable, and works as expected.

For instance — executing the test cases for the checkout process after a new discount feature is introduced, to ensure that the checkout process still works correctly and applies discounts to shopping carts as intended.

Here are the regression testing techniques:

Retest-all Regression Testing

The retest-all technique involves retesting the entire application; after code changes or bug fixes. Example: After fixing a bug in an e-commerce website’s checkout process, you test the entire checkout flow to ensure that the bug is resolved as per the bug report and no new bugs have crept in.

Corrective Regression Testing

Corrective regression testing focuses on verifying that specific defects have been fixed, without introducing new defects to the existing system. For example, suppose you’ve resolved a security vulnerability in a banking software. You perform corrective regression testing to confirm that the vulnerability is patched without affecting other functions.

Unit Regression Testing

Unit regression testing involves testing individual components (such as functions or methods) after code changes. Example: After modifying a critical function in an e-commerce app that calculates currency conversions during payment and checkout, you run unit regression tests specifically for that function to ensure it still works correctly.

Also Read: Unit Testing vs. Integration Testing

Regional Regression Testing

Regional regression testing focuses on specific areas or modules of the application affected by recent changes. Example: In an e-commerce app, if you’ve updated discount logic during a sale, you perform regional regression testing for the discount logic to verify its correctness.

Full Regression Testing

Full regression testing covers the end-to-end application, testing all features in unison. For example, before a major software version release, you execute full regression testing to ensure that all existing features work error-free and that new features haven’t affected the existing features in any way.

Also Read: End-to-End Testing Guide

While retesting might seem similar to regression testing, it's worth noting the nuances between the two. Regression testing focuses on re-running previously passed tests to ensure no regressions occurred, whereas retesting might involve re-running failed tests or specific functions. Let’s quickly compare them: