Simplified LDML to POSIX datetime format converter

snippet.js

/**
 * Simplified datetime format string converter from LDML (Locale Data Markup
 * Language) aka CLDR (Unicode Common Locale Data Repository) format to POSIX
 * aka strftime format. 
 *
 * Main usecase is using complete localization from CLDR with D3, which 
 * implements POSIX style of date formatting.
 *
 * References:
 *  - http://www.unicode.org/reports/tr35/tr35-dates.html#Date_Field_Symbol_Table
 *  - http://pubs.opengroup.org/onlinepubs/007908799/xsh/strftime.html
 *  - https://github.com/mbostock/d3/wiki/Time-Formatting#format
 *
 * @license LGPLv2.1+
 * @author saaj 
 */

var _ldmlToPosixMap = {
  // 03.09.1985 04:09:06 -04:00
  'y'    : '%Y',  // year: 1985
  'yy'   : '%y',  // year: 85
  'yyyy' : '%Y',  // year: 1985 
  'M'    : '%-m', // month: 9
  'MM'   : '%m',  // month: 09
  'MMM'  : '%b',  // month: Sept
  'MMMM' : '%B',  // month: September
  'd'    : '%e',  // day: 3
  'dd'   : '%d',  // day: 03
  'D'    : '%j',  // day: 246 
  'EEEE' : '%A',  // day: Tuesday
  'h'    : '%-I', // 12-hour: 4
  'hh'   : '%I',  // 12-hour: 04
  'H'    : '%-H', // 24-hour: 4 
  'HH'   : '%H',  // 24-hour: 04
  'm'    : '%-M', // minute: 9
  'mm'   : '%M',  // minute: 09
  's'    : '%-S', // second: 6 
  'ss'   : '%S',  // second: 06
  'a'    : '%p',  // AM or PM: AM
  'Z'    : '%Z',  // timezone: -0400
  'z'    : '%Z',  // only possible is -0400
  'zzzz' : '%Z'   // only possible is -0400
};

function convertLdmlToPosix(format)
{
  Object.keys(_ldmlToPosixMap)
    .sort(function(a, b)
    {
      // replace longest first
      return b.length - a.length;
    })  
    .map(function(key)
    {
      // Python dict.items
      return [key, _ldmlToPosixMap[key]];
    })
    .forEach(function(pair)
    {
      // avoid double replace, e.g. 'dd' -> '%d' -> '%%w'
      var regex = new RegExp('(%-?)?' + pair[0], 'g');
      format = format.replace(regex, function(match, percentGroup)
      {
        return percentGroup ? match : pair[1];
      });
    });
  return format;
}

console.assert(convertLdmlToPosix('EEEE, MMMM d, y') == '%A, %B %e, %Y');
console.assert(convertLdmlToPosix('MMMM d, y') == '%B %e, %Y');
console.assert(convertLdmlToPosix('MMM d, y') == '%b %e, %Y');
console.assert(convertLdmlToPosix('M/d/yy') == '%-m/%e/%y');

console.assert(convertLdmlToPosix('h:mm:ss a Z') == '%-I:%M:%S %p %Z');
console.assert(convertLdmlToPosix('h:mm:ss a Z') == '%-I:%M:%S %p %Z');
console.assert(convertLdmlToPosix('h:mm:ss a') == '%-I:%M:%S %p');
console.assert(convertLdmlToPosix('h:mm a') == '%-I:%M %p');