<?php

function minify_css($str){
    # remove comments first (simplifies the other regex)
    $re1 = <<<'EOS'
(?sx)
  # quotes
  (
    "(?:[^"\\]++|\\.)*+"
  | '(?:[^'\\]++|\\.)*+'
  )
|
  # comments
  /\* (?> .*? \*/ )
EOS;
    
    $re2 = <<<'EOS'
(?six)
  # quotes
  (
    "(?:[^"\\]++|\\.)*+"
  | '(?:[^'\\]++|\\.)*+'
  )
|
  # ; before } (and the spaces after it while we're here)
  \s*+ ; \s*+ ( } ) \s*+
|
  # all spaces around meta chars/operators
  \s*+ ( [*$~^|]?+= | [{};,>~+-] | !important\b ) \s*+
|
  # spaces right of ( [ :
  ( [[(:] ) \s++
|
  # spaces left of ) ]
  \s++ ( [])] )
|
  # spaces left (and right) of :
  \s++ ( : ) \s*+
  # but not in selectors: not followed by a {
  (?!
    (?>
      [^{}"']++
    | "(?:[^"\\]++|\\.)*+"
    | '(?:[^'\\]++|\\.)*+' 
    )*+
    {
  )
|
  # spaces at beginning/end of string
  ^ \s++ | \s++ \z
|
  # double spaces to single
  (\s)\s+
EOS;
    
    $str = preg_replace("%$re1%", '$1', $str);
    return preg_replace("%$re2%", '$1$2$3$4$5$6$7', $str);
}


$in = <<<'EOS'

p * i ,  html   
/* remove spaces */

/* " comments have no escapes \*/
body/* keep */ /* space */p,
p  [ remove ~= " spaces  " ]  :nth-child( 3 + 2n )  >  b span   i  ,   div::after

{
  /* comment */
    background :  url(  "  /* string */  " )   blue  !important ;
	content  :  " escapes \" allowed \\" ;
      width: calc( 100% - 3em + 5px ) ;
  margin-top : 0;
  margin-bottom : 0;
  margin-left : 10px;
  margin-right : 10px;
}

EOS;


$out = minify_css($in);

echo "input:\n";
var_dump($in);

echo "\n\n";
echo "output:\n";
var_dump($out);

?>