REGEX find and replace not working

I notice three possible errors in your initial expression.

First of all, you have to remove the string portion "<th " from the regular expression and ask DW to perform the search within the TH tag

Secondly, the error comes from the fact that your query is composed of a single string that will necessarily return only one result.
In fact, written like this, the words style and class are an integral part of your search, so you must isolate the pseudo values that would be assigned to them in order to consider them as the groups we are looking for.
The use of parentheses is therefore necessary to materialize this isolation.
So, instead of using the pattern

style="[^"]*" class="[^"]*"

It is therefore necessary to write

style="([^"]*)" class="([^"]*)"

This will give you the two returns distinctly identifiable by $1 and $2

Finally, this pre-request assumes that the two parts of the string are necessarily written in this order (style... then class..., and not class... then style....). For this reason, we will say that this is the case, and hope (for now) that it is the way it is.

But we also assume that the two parts are separated by only one space. This can be corrected by slightly modifying the query, and adding a [ ]*

So, instead of writing

style="([^"]*)" class="([^"]*)"

we can write

style="([^"]*)"[ ]*class="([^"]*)"

So, as a result, for the final REGEX in the search field you can write

style="([^"]*)"[ ]*class="([^"]*)"

And in the replacement field

style="$2" class="$1"

My English being what it is, I imagine that my explanations may not be very clear, but the code should work. Anyway, don't hesitate.