import { ValidatorFn } from "@angular/forms";
/**
 * Returns a validator function that tests a form control's value against a given regular expression.
 * If the value does not match the pattern, it returns a validation error with the provided error key.
 * Returns null for empty or valid values.
 *
 * @param pattern - The regular expression to test the control's value against.
 * @param errorKey - The key to use in the returned validation error object (defaults to 'patternInvalid').
 * @returns A ValidatorFn that returns a validation error object or null.
 */
export declare function patternValidator(pattern: RegExp, errorKey?: string): ValidatorFn;