Regex or string replace to add <p> html tag

Hi,
how can I use regex or string replace to add missing "p" tags to sentences without tags.
I tried matching* and splitting first the whole string matching "h" and "pre" tags but dont know how to merge it.

*let regexRule = /<pre>(.|\n|\r\n)[\s\S]*?<\/pre>/g;

Example - input

let someVariable = "Basket
<h1>Fruits</h1>
<pre>Apple
Juice</pre>
<pre>Kiwi</pre>
PVC thing
Trash
<h1>...</h1>";

How can I add "p" tag to Basket, PVC thing and Trash so output would be:

someVariable = "<p>Basket</p>
<h1>Fruits</h1>
<pre>Apple
Juice</pre>
<pre>Kiwi</pre>
<p>PVC thing</p>
<p>Trash</p>
<h1>...</h1>";

Thank you