I'm starting to think that maybe coffescript is actually okay... I would not have been able to wrap my primitive brain around this regular expression without it's block regex feature.

input.coffee

# An implementation of http://docs.python.org/2/library/string.html#format-specification-mini-language
# WARNING:  Not complete.

applyFormat = (value, formatSpec) ->
  pattern = ///
    ([^{}](?=[<>=^]))?([<>]^)? # fill & align
    ([\+\-\x20])? # sign
    (\#)? # integer base specifier
    (0)? # zero-padding
    (\d+)? # width
    (,)? # use a comma thousands-seperator
    (?:\.(\d+))? # precision
    ([bcdeEfFgGnosxX%])? # type
  ///
  [fill, align, sign, hash, zeropad, width, comma, precision, type] = formatSpec.match(pattern)[1..]
  if zeropad
    fill = "0"
    align = "="
  if ! align
    align = '>'

  switch type
    when 'b', 'c', 'd', 'o', 'x', 'X', 'n' # integer
      isNumeric = true
      value = '' + parseInt(value)
    when 'e','E','f','F','g','G','n','%' # float
      isNumeric = true
      value = parseFloat(value)
      if precision
        value = value.toFixed(parseInt(precision))
      else
        value = ''+value
    when 's' #string
      isNumeric = false
      value = '' + value

  if isNumeric && sign
    if sign in ["+"," "]
        if value[0] != '-'
          value = sign + value


  if fill
    while value.length < parseInt(width)
      switch align
        when '='
          if value[0] in "+- "
            value = value[0] + fill + value[1..]
          else
            value = fill + value
        when '<'
          value = value + fill
        when '>'
          value = fill + value
        when '^'
          throw new Error("Not implemented")

  return value

output.js

applyFormat = function(value, formatSpec) {
  var align, comma, fill, hash, isNumeric, pattern, precision, sign, type, width, zeropad, _ref, _ref1;
  pattern = /([^{}](?=[<>=^]))?([<>]^)?([\+\-\x20])?(\#)?(0)?(\d+)?(,)?(?:\.(\d+))?([bcdeEfFgGnosxX%])?/;
  _ref = formatSpec.match(pattern).slice(1), fill = _ref[0], align = _ref[1], sign = _ref[2], hash = _ref[3], zeropad = _ref[4], width = _ref[5], comma = _ref[6], precision = _ref[7], type = _ref[8];
  if (zeropad) {
    fill = "0";
    align = "=";
  }
  if (!align) {
    align = '>';
  }
  switch (type) {
    case 'b':
    case 'c':
    case 'd':
    case 'o':
    case 'x':
    case 'X':
    case 'n':
      isNumeric = true;
      value = '' + parseInt(value);
      break;
    case 'e':
    case 'E':
    case 'f':
    case 'F':
    case 'g':
    case 'G':
    case 'n':
    case '%':
      isNumeric = true;
      value = parseFloat(value);
      if (precision) {
        value = value.toFixed(parseInt(precision));
      } else {
        value = '' + value;
      }
      break;
    case 's':
      isNumeric = false;
      value = '' + value;
  }
  if (isNumeric && sign) {
    if (sign === "+" || sign === " ") {
      if (value[0] !== '-') {
        value = sign + value;
      }
    }
  }
  if (fill) {
    while (value.length < parseInt(width)) {
      switch (align) {
        case '=':
          if (_ref1 = value[0], __indexOf.call("+- ", _ref1) >= 0) {
            value = value[0] + fill + value.slice(1);
          } else {
            value = fill + value;
          }
          break;
        case '<':
          value = value + fill;
          break;
        case '>':
          value = fill + value;
          break;
        case '^':
          throw new Error("Not implemented");
      }
    }
  }
  return value;
};