discourse/app/assets/javascripts/pretty-text/engines/discourse-markdown/mentions.js.es6

function addMention(buffer, matches, state) {
  let username = matches[1] || matches[2];
  let tag = "span";
  let className = "mention";

  let token = new state.Token("mention_open", tag, 1);
  token.attrs = [["class", className]];

  buffer.push(token);

  token = new state.Token("text", "", 0);
  token.content = "@" + username;

  buffer.push(token);

  token = new state.Token("mention_close", tag, -1);
  buffer.push(token);
}

export function setup(helper) {
  helper.registerOptions((opts, siteSettings) => {
    opts.features.mentions = !!siteSettings.enable_mentions;
  });

  helper.registerPlugin(md => {
    const rule = {
      matcher: /@(\w[\w.-]{0,58}\w)|@(\w)/,
      onMatch: addMention
    };

    md.core.textPostProcess.ruler.push("mentions", rule);
  });
}