<!--(.*?)-->

For example, using the following regex would replace all p tags below with the required text.
<p class="some-para">(.*?)</p>

<p class="some-para"> Aabc asjfhhf </p>
<p class="some-para"> fgsg asjfhhf </p>
<p class="some-para"> Aabc ergherer </p>
<p class="some-para"> Aabergergerc asjfhhf </p>