$re = '/(\s?,\s?)(?=[^\(]*\))|(\s?,\s?)(?=(?:[^\(]*\([^\(]*?\))*\))/m';
$str = 'SELECT \'column1\' AS "Column 1" ,
\'column2\' AS "Column 2" ,
\'column3\' AS "Column 3",
SUM(Column_1, Column_2) , \'column5\' AS "Column 5" ,
CONCAT("str1","str2","str3") ,
ADD(len(str1),avg(column_1),sum(column_1,column_2,column_3)) ';
$subst = "$";
$result = preg_replace($re, $subst, $str);
echo "The result of the substitution is ".$result;
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php