Dummu - Testing Tip in Regex

A test that never fails is not testing anything, so this checks that it actually can.

Why this matters for Regex

This is a bold claim, an italic aside, and an underlined point, all in one Regex example.

What it touches:

• The Regex runtime or interpreter
• Whatever calls the code around it
• Anyone reading this six months from now

How to apply it:

1. Read the snippet below once, slowly.
2. Try changing one value and predict the output before running it.
3. Compare your prediction against what actually happens.

"A good test fails loudly the moment the behavior it protects changes."

'2026-08-26'.match(/^(\d{4})-(\d{2})-(\d{2})$/);
// ['2026-08-26', '2026', '08', '26']

Reference: Regex documentation

A good test fails loudly the moment the behavior it protects changes.