puyopuyotetr.is/app/javascript/mastodon/features/compose/util/counter.js

import { urlRegex } from './url_regex';

const urlPlaceholder = 'xxxxxxxxxxxxxxxxxxxxxxx';

export function countableText(inputText) {
  return inputText
    .replace(urlRegex, urlPlaceholder)
    .replace(/(?:^|[^\/\w])@(([a-z0-9_]+)@[a-z0-9\.\-]+[a-z0-9]+)/ig, '@$2');
};