$re = '/ (?(DEFINE)
(?<old> old [ ] string ) # (1)
)
(?|
( > ) # (2)
(?&old)
( < ) # (3)
|
( " ) # (2)
(?&old)
( " ) # (3)
|
( \' ) # (2)
(?&old)
( \' ) # (3)
|
( \( ) # (2)
(?&old)
( \) ) # (3)
)
/mx';
$str = '(old string) \'old string\' old string >old string<
"old string" >old string< old string \'old string\'
';
$subst = "$2new string$3";
$result = preg_replace($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