Regression Testing

Also Known As

TL;DR

Explanation

Regression testing encompasses all tests that verify known-good behaviour is preserved after a change. In CI/CD, the entire test suite is a regression suite — every commit runs all tests to catch regressions early. The cost of regression testing is proportional to test execution time, which is why the test pyramid (many fast unit tests, few slow E2E tests) optimises for fast regression feedback. Mutation testing extends regression testing by verifying that the tests themselves are meaningful.

Common Misconception

Why It Matters

Common Mistakes

• Not running regression tests on every commit — regressions compound when multiple unverified changes accumulate.
• Slow regression suite that developers skip locally — fast feedback loops require tests to run in minutes.
• Not adding a regression test when fixing a bug — without a test, the bug returns silently.
• Deleting tests that are 'in the way' of a refactor — they are protecting against regression.

Code Examples

// Bug fixed without adding a regression test:
// Report: calculate_discount() returns negative for items over $1000
// Fix applied — but no test added
// Next refactor: same bug reintroduced, silently
function calculate_discount(float $price): float {
    return $price > 100 ? $price * 0.1 : 0; // Bug: should be $price - $price * 0.1
}

// Bug fixed WITH a regression test:
function testDiscountNeverExceedsPrice(): void {
    $discount = calculate_discount(1500.00);
    $this->assertGreaterThanOrEqual(0, 1500.00 - $discount); // Price never goes negative
    $this->assertLessThanOrEqual(1500.00, $discount); // Discount never exceeds price
}

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