Character count for twitter. URLs are taken into consideration. Twitter 向けの簡易語数カウンタ。t.co による URL 圧縮も考慮します。

counting-tweet.js

function count_rest (text) {
  var limit = 140;

  // take t.co shorten into account
  // https://dev.twitter.com/blog/next-steps-with-the-tco-link-wrapper
  var regex  = /(?:https?\:\/\/|www\.)[^\s]+/g;
  var noURL  = text.replace(regex, "");
  var URLs   = text.match(regex);
  var length = noURL.length;

  if (URLs) {
    URLs.forEach(function (url) {
      if (url.match("https://")){
        length += 21;
      } else {
        length += 20;
      }
    });
  }

  return limit - length;
}

What's about emoji, other special chars?