Regular Expression :: Match Letter But Leave Html Entities Alone

I need to figure out a pattern that can match each letter of the message, but leaves all the html entities alone.

For example, I have a input like this:

<div>
This is the content &nbsp; &lt; Hello &gt;
</div>

Just as an example to make it more clearer, If I wanted to replace the all letters of the message with the number "1" I would have this result:

<div>
1111 11 111 1111111 &nbsp; &lt; 11111 &gt;
</div>