index.js

/**
 * Expose `isUrl`.
 */

module.exports = isUrl;

/**
 * RegExps.
 * A URL must match #1 and then at least one of #2/#3.
 * Use two levels of REs to avoid REDOS.
 */

var protocolAndDomainRE = /^(?:\w+:)?\/\/(\S+)$/;

var localhostDomainRE = /^localhost[\:?\d]*(?:[^\:?\d]\S*)?$/
var nonLocalhostDomainRE = /^[^\s\.]+\.\S{2,}$/;

/**
 * Loosely validate a URL `string`.
 *
 * @param {String} string
 * @return {Boolean}
 */

function isUrl(string){
  if (typeof string !== 'string') {
    return false;
  }

  var match = string.match(protocolAndDomainRE);
  if (!match) {
    return false;
  }

  var everythingAfterProtocol = match[1];
  if (!everythingAfterProtocol) {
    return false;
  }

  if (localhostDomainRE.test(everythingAfterProtocol) ||
      nonLocalhostDomainRE.test(everythingAfterProtocol)) {
    return true;
  }

  return false;
}