Regex Explainer — illustrative example

INPUT
JavaScript, no flags: ^[0-9]{4}$. Explain and test 2026, 026, 20A6 and 2026 followed by a newline.

RESULT
^ anchors the start; [0-9] allows ASCII digits; {4} requires exactly four; $ anchors the end.
2026 → true
026 → false
20A6 → false
2026 followed by a newline → false
This pattern checks shape, not whether a value is a valid year.

REVIEW
Explain the pattern in the correct regex flavor. Similar-looking syntax can behave differently across engines, especially for anchors and Unicode.

This is a teaching example, not a live execution record.