regression testing

Regression testing is the practice of re-running previously passing checks against software after a change to confirm that existing behavior still works. A regression is a defect in which a feature that used to work breaks, often as a side effect of a bug fix, an enhancement, a refactor, or a configuration change. It exists to catch that breakage before it reaches users.

Regression tests accumulate over a project’s life. A new test is typically added whenever a defect is found and fixed, so that the same fault can never silently return. The growing set is collected into a regression suite that runs repeatedly against new revisions, which makes automation and automated testing necessary as the suite scales.

Re-running the entire suite is thorough but slow, so several strategies trade coverage for speed:

  • Retest-all: Re-execute every test, giving full coverage at the highest cost.
  • Regression test selection: Run only the tests that exercise the changed code.
  • Test case prioritization: Order tests so the ones most likely to reveal a fault run first.

In practice, regression suites are wired into continuous integration, where each commit triggers a run that flags a regression before the change is merged.

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By Martin Breuss • Updated July 14, 2026 • Reviewed by Leodanis Pozo Ramos
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