Draft PHP script to join @media queries in a CSS file.

config.rb

# [..]

require 'shellwords'

on_stylesheet_saved do |filename|
  script = File.dirname(__FILE__) + '/_scripts/join-media-queries.php'
  system('php -q ' + Shellwords.escape(script) + ' -i ' + Shellwords.escape(filename))
end

join-media-queries.php

#!/usr/bin/env php
<?php

# Prepare the environment, be strict about silly mistakes.
error_reporting(E_ALL | E_STRICT);
ini_set('display_errors', 'On');

# Try and reset the memory limit imposed on CLI scripts.
ini_set('memory_limit', '64M');

# Handy function to format and print a string to STDERR and exit with a level.
function halt($message, $level, array $options = NULL) {
  file_put_contents('php://stderr', strtr($message, (array) $options) . PHP_EOL);
  exit(0 << $level);
}

# Start by making sure a file was passed as an argument to this script.
if (empty ($_SERVER['argc']) || $_SERVER['argc'] <= 1) {
  halt(<<<EOD
Usage: php -q :self [OPTION]... FILE

  -i[SUFFIX], --in-place[=SUFFIX]

    Edit files in place (makes backup if extension supplied)
EOD
, 1, array(
    ':self' => escapeshellarg($_SERVER['argv'][0]),
  ));
}

# Parse script options, e.g., '-i' for in-place updates.
$options = getopt('i::', array('in-place::'));

# The file always comes last.
$source = $_SERVER['argv'][$_SERVER['argc'] - 1];
$input   = NULL;
# Accept input from STDIN.
if ($source === '-') {
  $resource = fopen('php://stdin', 'rb');
  while ( ! feof($resource)) {
    $input = $input . fgets($resource, 4096);
  }
  fclose($resource);
} else {
  # The source is expected to be a file...
  if ( ! is_file($source)) {
    halt("[E] ':source' is not a file.", 2, array(
      ':source' => $source,
    ));
  }
  # ...which exists on disk...
  if ( ! file_exists($source)) {
    halt("[E] ':source' does not exist.", 3, array(
      ':source' => $source,
    ));
  }
  # ...and the User has sufficient permissions to access it for reading.
  if ( ! is_readable($source)) {
    halt("[E] ':source' cannot be read, permission denied.", 4, array(
      ':source' => $source,
    ));
  }

  # Read the entire file in one go. This could take a lot of memory for larger
  # buffers, but should be OK for the average sized CSS file.
  $input = file_get_contents($source);
}

# Recursively descend and match all `{ .. }` blocks.
preg_match_all('#(?<media>(?:@media[^{]+)?){(?:(?:[^{}]+)|(?R))*}#s', $input, $captures);

# Loop through all matches by key.
foreach (array_keys($captures[0]) as $key) {
  # If the value does not contain a @media query, drop it from the array. Or
  # even if we found a @media query, make sure it was not within a string or
  # a comment.
  if (strpos($captures[0][$key], '@media') === FALSE || empty ($captures['media'][$key])) {
    unset ($captures[0][$key]);
  }
}

# Erase all @media queries from the input CSS.
foreach ($captures[0] as $query) {
  $input = preg_replace('#\s*' . preg_quote($query, '#') . '#u', '', $input);
}

# Utility function to create an array key from a media query.
function sanitize_media_query_for_key($query) {
  $key = trim($query);
  $key = preg_replace('#[^\w\d]+#u', '-', $key);
  $key = trim($key, '-');
  return $key;
}

# For each @media query, group CSS code together.
$groups = array();
foreach (array_keys($captures[0]) as $key) {
  $media = $captures['media'][$key];
  # The code for this block starts off with the @media query in it...
  $code  = $captures[0][$key];
  # ...which then gets stripped off...
  $code  = preg_replace('#' . preg_quote($media, '#') . '#u', '', $code);
  # ...and any leading or trailing curly bracket also gets removed.
  $code  = preg_replace('#^{|}$#u', '', $code);
  # If this is the first time the @media query is encountered, a new group is
  # created.
  $group_key = sanitize_media_query_for_key($media);
  array_key_exists($group_key, $groups) OR ($groups[$group_key] = array($media, '{'));
  $groups[$group_key][] = $code;
}

$result = NULL;
# Output is the input stripped of any @media queries...
$result = $result . rtrim($input);
# ...followed by those @media queries grouped together.
foreach (array_keys($groups) as $key) {
  $result = $result . PHP_EOL . PHP_EOL . implode(NULL, $groups[$key]) . '}';
}

# If --in-place was specified, print result to a file.
if (array_key_exists('i', $options) || array_key_exists('in-place', $options)) {
  # If we read from STDIN, assume option value is the file name.
  if ($source === '-') {
    $source = NULL;
  }
  file_put_contents($source . (isset ($options['i']) ? $options['i'] : $options['in-place']), $result);
} else {
  print $result;
}

On large files we get this PHP Error.

Warning: preg_replace(): Compilation failed: regular expression is too large at offset 41501 in PATH/sass/_scripts/join-media-queries.php on line 89

These parameter changed nothing.

...
// Try and reset the memory limit imposed on CLI scripts.
ini_set('memory_limit', '512M');
// essential for huge PCREs
ini_set("pcre.backtrack_limit", "23001337");
ini_set("pcre.recursion_limit", "23001337");
// imagine your PCRE here...
...

Do you have a solution for this?

Nice one thank you!
How about sorting the media queries?