regex replace: search pattern from a variable?

I'd like to do a regex_replace where both the search & replace values are stored in variables. (It's a whole lot neater and useful.)

Storing the replace text in a variable works fine, but not the search text. I'm probably just making an ignorant mistake.

The following is the working code from my Entry template.

<mt:Assets> <mt:AssetLabel setVar="my_asset_label" /> <mt:if name="body_content" like="$my_asset_label"> <mt:SetVarBlock name="new_image_block"> <div class="std_side_image_$4" style="width: 150px;"> <img src="$3" width="" height="" alt="[PHOTOGRAPH] $2" title="[PHOTOGRAPH] $2"> <p>$2</p> </div> </mt:SetVarBlock> </mt:if> </mt:Assets>

<mt:var name="body_content"
regex_replace="/<span class="mt-enclosure .+?><a href="(.+?)"><img alt="(.+?)" src="(.+?)" class="mt-image-(.+?)".+?</span>/g","$new_image_block"
/>