$re = '/^(?:(“(?! “).*?)\s*”|(“) “(.*)|((?!“).*?)\s*”)$/m';
$str = '“ Criteria 1, ending with RD and beginning with LD, but not LDLD, remove RD ”
“ “ Criteria 1, ending with RD but beginning with LDLD, do nothing to RD ”
“ “ Criteria 2, beginning with LDLD, make it begin with LD ”
Criteria 3 with non-matching RD, remove RD ”';
$subst = "$1$2$3$4";
$result = preg_replace($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