$re = '/("[^"]*)ABC([^.][^"]*"|")/mis';
$str = 'BLABLUB SHSJSKSKDKDNDNDK hD ABC mmmjkk "hshs ABC.TTTTT" dndjmsmdmdmdmdm "blablub ABC blablub" khhg "zzggj ABC"';
$subst = "\1DEF\2";
$result = preg_replace($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