$1 not working with sed

I have a bunch of files that contain XML tags like:

<h> PIDAT <h> O

I need to delete everything what comes after the first <h> in that line, so I can get this:

<h>

For that I'm using

sed -i -e 's/(^<.*?>).+/$1/' *.conll

But it seems that sed is not recognizing the $1. (As I understand, $1 should delete everything what is not contained in the group). Is there a way I can achieve this? I'd really appreciate if you could point me in the right direction.

PS: I tested those expressions on a regex app and they worked, but it is not working from the command line.