/**
 * Validates if the given identifier could be a PubMed ID (PMID) or PubMed Central ID (PMCID).
 *
 * @param {string} identifier - The identifier string to validate.
 * @returns {boolean} `true` if the identifier could be a valid PMID or PMCID, `false` otherwise.
 */
declare const _default: (identifier: string | undefined) => boolean;
export default _default;