showdownjs/src/subParsers/headers.js

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

  var prefixHeader = options.prefixHeaderId;

  // Set text-style headers:
  //	Header 1
  //	========
  //
  //	Header 2
  //	--------
  //
  text = text.replace(/^(.+)[ \t]*\n=+[ \t]*\n+/gm, function (wholeMatch, m1) {

    var spanGamut = showdown.subParser('spanGamut')(m1, options, globals),
        hID = (options.noHeaderId) ? '' : 'id="' + headerId(m1) + '"',
        hashBlock = '<h1' + hID + '>' + spanGamut + '</h1>';
    return showdown.subParser('hashBlock')(hashBlock, options, globals);
  });

  text = text.replace(/^(.+)[ \t]*\n-+[ \t]*\n+/gm, function (matchFound, m1) {
    var spanGamut = showdown.subParser('spanGamut')(m1, options, globals),
        hID = (options.noHeaderId) ? '' : 'id="' + headerId(m1) + '"',
        hashBlock = '<h2' + hID + '>' + spanGamut + '</h2>';
    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})				// $1 = string of #'s
   [ \t]*
   (.+?)					// $2 = Header text
   [ \t]*
   \#*						// optional closing #'s (not counted)
   \n+
   /gm, function() {...});
   */

  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(m1) + '"',
        header = '<h' + m1.length + hID + '>' + span + '</h' + m1.length + '>';

    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;
});