/**
 * Performs a syntactic check that a string looks like an email address.
 *
 * Aligned with the HTML5 / WHATWG `<input type="email">` definition and the
 * RFC 5321 size limits:
 *
 * - Local part: any of `a-z A-Z 0-9 .!#$%&'*+/=?^_\`{|}~-`, 1–64 chars, with
 *   no leading/trailing dot and no consecutive dots.
 * - Domain: dot-separated labels of `a-z A-Z 0-9 -`, each 1–63 chars and not
 *   starting or ending with a hyphen; total address length ≤ 254 chars.
 *
 * This is a best-effort syntactic check — it is not the full RFC 5322 grammar
 * (no quoted local parts, no IP-literal or IDN/Unicode domains) and does not
 * confirm the mailbox exists. For authoritative validation, send a
 * confirmation email.
 *
 * @param value - The string to test.
 * @returns `true` if `value` matches the email shape; otherwise `false`.
 *
 * @example
 * ```ts
 * isEmailValid("user@example.com");      // true
 * isEmailValid("user.name+tag@host.co"); // true
 * isEmailValid("user@host.museum");      // true
 * isEmailValid("not-an-email");          // false
 * isEmailValid(".user@example.com");     // false (leading dot)
 * isEmailValid("a..b@example.com");      // false (consecutive dots)
 * ```
 */
export declare function isEmailValid(value: string): boolean;