I use now the stripslashes function,here is my code

<form action='regex.php' method=post>
String:
<input type=text name='string' value=<?echo stripslashes($string);?>><br>
<input type=submit value=ok><br>
<?php
  $str = stripslashes($str);
  echo "$str<br>";
?>
</form>

when I only write '"string' without '"' at the end in <input..>, you should see what in <input..> stand .
But when I write '"string"' or 'string"', it is normal .

That's all ..