Split into arrays, the difficulty is C (x, y ("Z", 2, 0)), is a whole.

The string $s = ' A, B, C (x, y ("Z", 2, 0)), D, E ';
Split into arrays, the difficulty is C (x, y ("Z", 2, 0)), is a whole.

Desired Result:
Array
' A ',
' B ',
' C (x, y ("Z", 2, 0)) ',
' D ',
' E ');

Reply to discussion (solution)

$s = ' A, B, C (x, y ("Z", 2, 0)), D, E '; $keywords = Preg_split ("/\,\s (? =[a-z])/", $s); Var_dump ($keywords);

$s = ' A, B, C (x, y ("Z", 2, 0)), D, E '; $keywords = Preg_split ("/\,\s (? =[a-z])/", $s); Var_dump ($keywords);

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Thank you very much for your reply! But relying on the =[a-z] is inaccurate, because it is possible that the parentheses are also uppercase (within the nested parentheses, whatever the content is a whole).
$s = ' A, B, C (X, Y ("Z", 2, 0)), D, E ';

$s = ' A, B, C (x, y ("Z", 1, 0)), D (x, y ("Z", 2, 0)), e,f (x, y ("Z", 3, 0)) ';//extract Get inside Contents Preg_match_all ("/\ (. *?\) \)/", $s, $match) ;//This part can be modified by itself $s = str_replace ($match [0], '% ', $s);//will be replaced as a whole,% percent sign can also be selected $exs = Explode (', ', $s); $i = 0;foreach ($exs as $key = = $value) {if (Strpos ($value, "%")!== false) {$exs [$key] = str_replace ('% ', $match [0][$i], $value); $i + +;}} Var_dump ($EXS);

To help you complicate it.

So this is not the right thing to do, and honestly write a function than the struggle to write a regular more affordable

 $s = ' A, B, C (x, Y ("Z", 2, 0)), D, E ';p rint_r (foo ($s)), function foo ($s) {$r = array ();  $m = 0;  $t = "; for ($i =0; $i 
   
   
array ([0] = A [1] = B [2] = C (x, Y ("Z", 2, 0)) [3] =  > D [4] + E) 
 
   
 
 
   
 thanks to 2-bit reply. 
 think of a matching parenthesis nested regular, with similar calculations can refer to.  (no rigorous testing) 
 
 $s = ' A, B,c (X, Y ("Ez", 2, 0, Z (KK, a))), D, E (U (8, D (88))) '; 
 Preg_match_all ('/[a-z] (? =[,]) |[ ^,]*\(([^()]+| (?  R)) *\)/', $s, $z);
  
 Print_r ($z [0]); 
 
   
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