pattern()
🧭 API map
| I want to… | Details |
|---|---|
| See every accepted call style | Signatures |
| See common and advanced usage | Usage and behavior |
| Customize messages | Message configuration |
| Understand reactive constraints | Reactive behavior |
| Return to the complete catalog | Built-in validators |
📐 Signatures
pattern(expression)
pattern(expression, message)
pattern(expression, options)
Reactive constraint arguments use a zero-argument function. The function may read signals and,
where supported, return undefined to disable the constraint temporarily.
📖 Usage and behavior
Requires a non-empty string to match a regular expression:
const myForm = form({
code: field('', [pattern(/^[A-Z]{3}-\d{4}$/)]),
username: field('', [pattern(() => configuredPattern())]),
countryCode: field('', [pattern(/^[A-Z]+$/, 'Use letters only.')]),
});
null and '' pass. A reactive expression returning undefined disables the constraint. The expression's lastIndex is reset before every check, so global and sticky regular expressions do not reuse stale match state. A failure is { kind: 'pattern', pattern, actual, message }. Every active expression appears in pattern() metadata.
💬 Message configuration
Every failure has a default English message. Where supported, pass a string as the final argument or use an options object for a static or reactive message, as shown above.
A message function may read signals. Returning undefined continues through node, Angular
provider, process-wide, and built-in message fallbacks. See
Validator messages.
⚡ Reactive behavior
The options object accepts a reactive when predicate. Signals read from its validator context are
tracked; while it returns false, the rule contributes neither errors nor constraint metadata.
const validateReference = signal(false);
const reference = field('123', [
pattern(/^REF-\d+$/, { when: () => validateReference() })
]);
Reactive constraint functions and message functions track the signals they read. When a resolved constraint becomes unavailable, validators that support optional constraint sources temporarily stop contributing their error and metadata.
The validator runs synchronously as part of its node's validator source. Disabled, readonly, and hidden nodes skip validation until they become interactive again.