$re = '/(str)(?=(?:[^\'"]|["\'][^\'"]*["\'])*$)/';
$str = 'test str , a "comma, si str", bobsi str "si,sam, str",here si str \' hh str si\'';
$subst = "newstr";
$result = preg_replace($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