showdownjs/src/subParsers/makehtml/italicsAndBold.js

showdown.subParser('makehtml.italicsAndBold', function (text, options, globals) {
  'use strict';

  text = globals.converter._dispatch('makehtml.italicsAndBold.before', text, options, globals);

  // it's faster to have 3 separate regexes for each case than have just one
  // because of backtracing, in some cases, it could lead to an exponential effect
  // called "catastrophic backtrace". Ominous!

  function parseInside (txt, left, right) {
    /*
    if (options.simplifiedAutoLink) {
      txt = showdown.subParser('simplifiedAutoLinks')(txt, options, globals);
    }
    */
    return left + txt + right;
  }

  // Parse underscores
  if (options.literalMidWordUnderscores) {
    text = text.replace(/\b___(\S[\s\S]*)___\b/g, function (wm, txt) {
      return parseInside (txt, '<strong><em>', '</em></strong>');
    });
    text = text.replace(/\b__(\S[\s\S]*)__\b/g, function (wm, txt) {
      return parseInside (txt, '<strong>', '</strong>');
    });
    text = text.replace(/\b_(\S[\s\S]*?)_\b/g, function (wm, txt) {
      return parseInside (txt, '<em>', '</em>');
    });
  } else {
    text = text.replace(/___(\S[\s\S]*?)___/g, function (wm, m) {
      return (/\S$/.test(m)) ? parseInside (m, '<strong><em>', '</em></strong>') : wm;
    });
    text = text.replace(/__(\S[\s\S]*?)__/g, function (wm, m) {
      return (/\S$/.test(m)) ? parseInside (m, '<strong>', '</strong>') : wm;
    });
    text = text.replace(/_([^\s_][\s\S]*?)_/g, function (wm, m) {
      // !/^_[^_]/.test(m) - test if it doesn't start with __ (since it seems redundant, we removed it)
      return (/\S$/.test(m)) ? parseInside (m, '<em>', '</em>') : wm;
    });
  }

  // Now parse asterisks
  if (options.literalMidWordAsterisks) {
    text = text.replace(/([^*]|^)\B\*\*\*(\S[\s\S]+?)\*\*\*\B(?!\*)/g, function (wm, lead, txt) {
      return parseInside (txt, lead + '<strong><em>', '</em></strong>');
    });
    text = text.replace(/([^*]|^)\B\*\*(\S[\s\S]+?)\*\*\B(?!\*)/g, function (wm, lead, txt) {
      return parseInside (txt, lead + '<strong>', '</strong>');
    });
    text = text.replace(/([^*]|^)\B\*(\S[\s\S]+?)\*\B(?!\*)/g, function (wm, lead, txt) {
      return parseInside (txt, lead + '<em>', '</em>');
    });
  } else {
    text = text.replace(/\*\*\*(\S[\s\S]*?)\*\*\*/g, function (wm, m) {
      return (/\S$/.test(m)) ? parseInside (m, '<strong><em>', '</em></strong>') : wm;
    });
    text = text.replace(/\*\*(\S[\s\S]*?)\*\*/g, function (wm, m) {
      return (/\S$/.test(m)) ? parseInside (m, '<strong>', '</strong>') : wm;
    });
    text = text.replace(/\*([^\s*][\s\S]*?)\*/g, function (wm, m) {
      // !/^\*[^*]/.test(m) - test if it doesn't start with ** (since it seems redundant, we removed it)
      return (/\S$/.test(m)) ? parseInside (m, '<em>', '</em>') : wm;
    });
  }


  text = globals.converter._dispatch('makehtml.italicsAndBold.after', text, options, globals);
  return text;
});