showdown/src/subParsers/italicsAndBold.js

showdown.subParser('italicsAndBold', function (text) {
  'use strict';
  // <strong> must go first:
  text = text.replace(/(\*\*|__)(?=\S)([^\r]*?\S[*_]*)\1/g, '<strong>$2</strong>');

  text = text.replace(/(\*|_)(?=\S)([^\r]*?\S)\1/g, '<em>$2</em>');

  return text;
});