showdownjs/src/subParsers/headers.js

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

  var prefixHeader = options.prefixHeaderId,
      headerLevelStart = (isNaN(parseInt(options.headerLevelStart))) ? 1 : parseInt(options.headerLevelStart),

  // Set text-style headers:
  //	Header 1
  //	========
  //
  //	Header 2
  //	--------
  //
      setextRegexH1 = (options.smoothLivePreview) ? /^(.+)[ \t]*\n={2,}[ \t]*\n+/gm : /^(.+)[ \t]*\n=+[ \t]*\n+/gm,
      setextRegexH2 = (options.smoothLivePreview) ? /^(.+)[ \t]*\n-{2,}[ \t]*\n+/gm : /^(.+)[ \t]*\n-+[ \t]*\n+/gm;

  text = text.replace(setextRegexH1, function (wholeMatch, m1) {

    var spanGamut = showdown.subParser('spanGamut')(m1, options, globals),
        hID = (options.noHeaderId) ? '' : ' id="' + headerId(m1) + '"',
        hLevel = headerLevelStart,
        hashBlock = '<h' + hLevel + hID + '>' + spanGamut + '</h' + hLevel + '>';
    return showdown.subParser('hashBlock')(hashBlock, options, globals);
  });

  text = text.replace(setextRegexH2, function (matchFound, m1) {
    var spanGamut = showdown.subParser('spanGamut')(m1, options, globals),
        hID = (options.noHeaderId) ? '' : ' id="' + headerId(m1) + '"',
        hLevel = headerLevelStart + 1,
      hashBlock = '<h' + hLevel + hID + '>' + spanGamut + '</h' + hLevel + '>';
    return showdown.subParser('hashBlock')(hashBlock, options, globals);
  });

  // atx-style headers:
  //  # Header 1
  //  ## Header 2
  //  ## Header 2 with closing hashes ##
  //  ...
  //  ###### Header 6
  //
  text = text.replace(/^(#{1,6})[ \t]*(.+?)[ \t]*#*\n+/gm, function (wholeMatch, m1, m2) {
    var span = showdown.subParser('spanGamut')(m2, options, globals),
        hID = (options.noHeaderId) ? '' : ' id="' + headerId(m2) + '"',
        hLevel = headerLevelStart - 1 + m1.length,
        header = '<h' + hLevel + hID + '>' + span + '</h' + hLevel + '>';

    return showdown.subParser('hashBlock')(header, options, globals);
  });

  function headerId(m) {
    var title, escapedId = m.replace(/[^\w]/g, '').toLowerCase();

    if (globals.hashLinkCounts[escapedId]) {
      title = escapedId + '-' + (globals.hashLinkCounts[escapedId]++);
    } else {
      title = escapedId;
      globals.hashLinkCounts[escapedId] = 1;
    }

    // Prefix id to prevent causing inadvertent pre-existing style matches.
    if (prefixHeader === true) {
      prefixHeader = 'section';
    }

    if (showdown.helper.isString(prefixHeader)) {
      return prefixHeader + title;
    }
    return title;
  }

  return text;
});