Regression Testing vs Functional Testing: How Each Impacts Quality and When to Use Them

“How can we speed up delivery?” is a pretty dangerous question. And yet, rising competition and shifting user expectations make us come back to that query again and again. Process optimization, investing in quality tools, and advancing team skills would be an unrivaled mix of solutions. But in some cases, they’d take too long, be a tad expensive, or turn out impractical. These situations lead to another scary question: “Should we skip something?”.

And quite often, a crew’s eyes land on regression vs functional testing. They’re utterly similar, after all. While omitting certain elements of quality assurance is not unheard of, even NASA does it, this approach shouldn’t be applied to the mentioned duo. Today, we’ll explore:

• Why that’s the case.
• How to use both functional and regression testing.
• And what can help you up project quality and stress less about expenses and effort.

What Is Functional Testing?

Let’s start by figuring out: what is functional testing? Skipping mind-bending technical terms, functional tests aim to verify that your app does what it’s supposed to. You want to make sure that every feature in your product works correctly, focusing on requirements and user behavior (what an actual person will do).

Typical Scenarios Functional Testing Covers

One thing you should remember about functional testing is that it’s sort of an umbrella term. As you know, many testing types help figure out whether something works as needed.

• Sanity testing checks specific features or bug fixes after a minor update.
• Integration testing checks how different modules or components operate together.
• System testing checks the entire application as a whole.
• User acceptance testing (UAT) checks whether the system meets user expectations.
• Localization testing checks feature behavior in different languages or regions.

All of the above ask the same question: “Does this work?”. They simply look at a matter from different perspectives. So, functional testing is both a general category and a specific testing activity. That’s a little confusing. But hey, nothing about QA is really straightforward. Just keep in mind that functional testing services are exceptionally multifaceted. That’s why they can cover literally thousands of scenarios:

• Run a quick check to confirm a newly deployed feature loads and doesn’t break the app (smoke testing).
• Validate that different user roles (admin, viewer) can only access authorized features (system testing).
• Submit a form and verify that the backend API returns the correct response and updates the database (integration testing).
• Business users test the entire task creation workflow to confirm it meets business needs (UAT testing).
• Click through paths between dashboard, settings, and profile pages to check if they behave as expected (system testing).
• Ensure that required fields, formats, and error messages behave correctly in all forms (general functional testing).
• Confirm that switching to Spanish updates all labels and doesn’t break layout or logic (localization testing).
• Verify that sessions expire after a set time and redirect the user appropriately (general functional testing).
• After backend changes, verify that login still works with valid and invalid credentials (regression testing). That’s a little spoiler for you.

Now that you know what functional testing does for your software, let’s review its impact on your business.

Business Benefits of Functional Testing

Think of the advantages as reasons not to cut off too much from functional testing. A few “trims” here and there might save you time and money. But they might just chop off your quality. Realistically, functional tests are the backbone of your project. You first establish whether something works. Then, you move on to how it works, when it works, etc. Can’t have it any other way.

That’s why functional testing has such a strong impact on your project; it:

1. Ensures features work as intended, leading to smoother interactions and happier users.
2. Catches defects before the software reaches users, minimizing disruptions and support costs.
3. Helps fix issues early, saving money compared to post-release patches and preventing customer frustration.
4. Validates that software delivers on defined requirements, supporting strategic objectives.
5. Builds trust in new versions, enabling faster and safer deployment cycles.
6. Secures consistently working features, contributing to a strong brand image and competitive advantage.
7. Helps ensure software meets necessary regulations and contractual obligations.

Now, objectively, having a “functioning software” isn’t anything phenomenal. Users don’t want an app that works just fine. They want something fun, intuitive, and pleasant. Most of the time, these traits are achieved through non-functional tests, which center on performance, stability, usability, etc.

Does that mean that these types of testing are more important? No. Because if your app is extremely enjoyable, yet it breaks, glitches, or freezes, then nothing else matters. Functional testing is the skeleton atop which you can build. Without it, you’ll just have a wobbly structure that’s frankly useless (imagine a human without bones).

That’s the real value of functional testing—it creates a foundation which you can turn into something amazing.

What Is Functional Regression Testing?

Functional regression testing is interesting because it kind of does the same thing as our previous player—checks whether something works. To properly answer “what is regression testing?”, we’ll have to take a few detours. So, buckle up.

Regression testing checks whether recent changes have altered or broken existing functionality. Unlike functional tests, regression looks at the bigger picture. It doesn’t focus on the exact feature that was modified. It also reviews elements that could have been impacted by fixes or updates. And here’s why it’s important.

Imagine that your team updated the user profile editing feature to allow adding more than one phone number. This addition can have multiple ripple effects:

• The notifications system might break because it only expects one phone number and now can’t handle multiples.
• The security settings page might fail to validate phone numbers correctly, causing errors when users try to save changes.
• The account recovery process might still send recovery codes to the old phone number instead of the new ones.
• The billing system that sends SMS alerts could crash or send messages to the wrong numbers.

So, yes, your new feature technically works. But others that connect to it might fail. And so, what regressions testing services do is check for unintended side effects in your app.

Typical Scenarios Functional Regression Testing Covers

Regression tests, due to their nature, can also touch upon thousands of cases. Here are a few examples:

• Verify adding, updating, and removing items from the cart works correctly after a new checkout feature is added.
• Check that all required fields validate properly and forms submit successfully after UI tweaks.
• Confirm that payments still go through and receipts generate correctly after backend updates.
• Make sure password reset emails send and reset links work after security patches.
• Validate that product or content search returns correct results after database optimizations.
• Test that users can update their details and changes persist after server alterations.
• Ensure push notifications and email alerts trigger correctly after integration with third-party messaging services.
• Verify that menu items and links lead to the correct pages after UI redesigns.
• Check that reports export and display accurate data after changes in reporting logic.

You’ve already noticed regression testing’s specialty—it’s run after modifications. So, the big difference between functional and regression testing is timing. First, you execute functional tests to see if something works. Then, you execute regression tests to see if it still works. This separation may seem redundant. But it actually brings convenience to QA teams, defining the purpose of a test or suite and helping manage the testing process.

Business Benefits of Functional Regression Testing

The impact of regression testing greatly overlaps with that of functional testing. So, here, we’ll focus on its unique perks.

1. Regression testing ensures new updates don’t break previously working features, protecting user trust and retention.
2. Catching bugs introduced by recent changes early helps avoid costly urgent fixes and service disruptions.
3. Regression tests safeguard the value built into legacy functionality as the software evolves, ensuring business-critical features remain reliable.
4. Their results also inform release readiness, helping prioritize fixes and avoid risky deployments.
5. Regression testing ensures that changes don’t inadvertently break compliance features or security safeguards.
6. It supports agile workflows by validating that ongoing changes don’t disrupt stable parts of the product, accelerating release cycles.
7. Consistent product quality and stability reduce negative user experiences that can damage brand perception.

A simpler way to look at it is that regression testing is about making sure that the work done was done well. Because regression runs can also help you advance your QA process. For example, frequent bugs in the same areas or a high number of missed defects can mean that prior activities weren’t up to par. Maybe there was fragile code, insufficient test coverage, or gaps in test planning. And so, regression testing can also act as a phase for analyzing present efforts.

Why Are Functional Testing & Regression Testing Lumped Together?

This section will have a bit of brainstorming to help hammer down the distinctions between functional testing vs regression testing.

Similarities That Lead to Confusion

Both regression and functional testing check whether software features behave as expected. But the former focuses on the side effects of changes. And the latter centers on initial implementation bugs.

This is not the only case of “it’s the same thing” in development and QA. A similar situation is happening to automated vs continuous testing; functional, regression, and smoke testing; and even unit testing vs functional testing vs regression testing. Call it obscure terminology or overcomplication, but we have what we have. So, let’s break it down.

• Functional testing covers functional aspects of software (feature behavior) and includes multiple types of checks.
• Regression testing looks for any errors that changes might have introduced.
• Smoke testing is a quick, high-level investigation that makes sure your build isn’t in shambles. It assesses if the core features work, signaling that it’s okay to proceed with other tests.
• Unit testing ensures that the smallest units of code behave correctly on their own. It’s usually run by developers and is entirely code-focused.

Regression, smoke, and unit testing can be grouped under functional testing as they check that something works. The difference comes from what they check.

Non-Functional & Functional Regression Testing

If you’ve noticed, in the previous sections, we’ve used a lot of “functional regression testing”. That’s not a whim.

• Any type of functional test is about what software does (features, behaviors, and user actions).
• And any type of non-functional test is about how software does it (speed, scalability, security, usability, reliability).

So, is regression testing functional or non-functional? It can be both. When you test a non-functional aspect of an app, regression becomes non-functional. Pardon the tautology. Here’s an illustration.

• After updating the login logic, you confirm that users can still reset their password and access the dashboard. That’s a functional test (does it work).
• After a backend optimization, you make sure page load times don’t get slower. That’s a non-functional test (how it works).

Integration, localization, and even usability testing can be non-functional. It’s all a matter of context.

Differences between Functional & Regression Testing to Remember

Let’s finish up our comparison by summarizing key distinctions between functional and regression testing. Consider this your cheat sheet.

Given that functional testing and regression testing fulfil different purposes, you don’t want to substitute one for the other. There’s no “versus” between these two. They should go hand in hand and complement each other. And moderating or skipping either will only lead to a huge dip in quality.

Regression Testing vs Functional Testing: When to Use Which

With the above in mind, let’s break down when you should turn to either functional or regression testing. It’s critical to know when to rely on each since mismatches can lead to lost time, team productivity, and money. Maybe QA and dev terms are often confusing because it motivates people to really dig into the topic. After all, without proper knowledge or skills, quality is simply unachievable. That’s something you should always remember.

Now, take a minute to take in this quick guide.

Functional testing is used when:

• A new feature has been developed, and it needs to be validated against requirements.
• A component or function has been redesigned or rebuilt.
• You need to check if a system meets user or business expectations.
• A client or stakeholder wants to verify specific behavior before sign-off.
• You’re testing for compliance with specs, rules, or functional standards.
• A system is integrated with new services or APIs, and you want to verify expected interactions.
• You’re confirming that user-facing actions (e.g., submitting a form, placing an order) work as intended.
• You need to validate variations in input/output behavior, especially for edge cases.
• The software is being customized or localized, and you want to ensure new functional rules work.
• You’re running acceptance tests to verify whether the software is ready for production.

Regression testing is used when:

• Any code change is introduced.
• A bug is fixed, and you want to ensure the fix didn’t break anything else.
• A new feature has been added, and you want to make sure the existing functionality still works.
• You’re preparing for a release and need to ensure software stability.
• There’s refactoring or optimization of existing code.
• A platform update, dependency upgrade, or system-level change might affect application behavior.
• An automation suite is regularly run as part of continuous integration to catch side effects early.
• You’re doing routine QA to guard against regressions across multiple product versions.
• The product has gone through multiple iterations, and you want to catch hidden breakages.
• A user reports a bug in a different area after recent changes, suggesting an unintended side effect.

So, functional testing is for working with new features or updates. And regression testing it for verifying that recent changes haven’t affected present functionality.

Automation Feasibility for Functional vs Regression Testing

Both functional and regression testing are great contenders for automation. These testing types are run frequently, and there are always lots of tests involved. But, to be honest, things are much simpler with regression.

Not all functional checks are equally suited for automation. Tests that cover stable, high-usage features or core workflows pair well with it. You might also consider automating tests that need repetitive execution or consistent input validation.

On the other hand, scenarios that rely heavily on complex user interaction, visual validation, or changing requirements often remain more efficient when performed manually.

Compared to functional testing, regression is simply the perfect candidate for automation. Here’s why. Functional tests are always executed manually first. They’re also often reused as regression tests, so there’s less effort involved—you already have something to start with. Similarly, you run regression checks on elements that have already been investigated and are considered stable. So, big changes or sudden failures are unlikely. And thus, maintaining regression tests is easier.

Automated regression testing definitely offers greater ROI. But that doesn’t mean that you should ignore automating functional tests. Remember that automated testing is about efficiency. So, there’s really no point in depriving your functional checks of it. Yet, if you’re on a tight budget, timeline, or skilled experts, we’d recommend focusing your automation efforts on regression.

Alternatively, you could turn to outsourced QA. With it, you get instant access to experienced specialists with diverse skills. They can help you with pretty much anything regarding functional and regression testing.

• Identify missing or weak functional test scenarios.
• Create clear, structured test cases that align with business requirements.
• Map out regression suites that evolve with your product lifecycle.
• Provide bandwidth to execute large-scale functional or regression test cycles.
• Run parallel tests across different platforms, environments, or configurations.
• Recommend what to automate and what to leave manual based on ROI and stability.
• Set up and maintain automation frameworks and select functional and regression testing tools.
• Build and maintain reusable automated suites.
• Handle ongoing automation script updates as features evolve.
• Refactor brittle or outdated scripts to reduce test flakiness and false positives.
• Continuously review and update regression packs to keep them lean and relevant.
• Implement test automation within CI/CD pipelines.
• Introduce reusable components, modular test architecture, and proper version control.

Partnering up with a QA company also offers many advantages for your team, product, and business. External expertise can unload your crew and let them focus on other critical tasks. They can offer a fresh, unbiased perspective, which will refine your app. And you can avoid the cost of recruiting and training an in-house QA team for temporary needs.

The combination of the above factors gives your product a considerable quality boost. And, in our humble opinion, that’s the true power and the biggest perk of QA services.

To Sum Up

Both regression and functional testing are paramount for your journey toward a remarkable product. They aren’t something to cut back on. And if you’re in a situation where you’re really considering it, external expertise is a zero-flaw solution. So, take some time to assess your QA capabilities. And if you see that a certain aspect might prevent you from creating a project users will love—reach out to specialists. Quality isn’t to be frugal with. Invest in it.