Help with form and post!

Hope you can help me :P

I'm coding an html form with some php vars. Something like this doesn't work:

<form action="./correct.php" method="post">
(xxxxx) <input type="hidden" name="preg1" value="'. $preg .'">
<input submit.....>
</form>

And then in correct.php:
echo $_POST["preg1"]

But if I substitute the line (xxxxx) with this it works!
<input type="hidden" name="preg1" value="sometexthere">

What can I do to make it works using $preg var??

Thanks and regards
.