Hi,
First note:
LoadUIWeb is an outdated product by SmartBear that was rebranded to LoadComplete (https://smartbear.com/product/loadcomplete/overview/). The current version is 4.50 and it was significantly improved since the times of LoadUIWeb. As LoadUIWeb is not supported for several years, there are no chances for some patch even if you really found a problem.
LoadComplete up to v.4.40 had a free version which was discontinued since v.4.50. But you may try to search for it or contact SmartBear Sales via https://support.smartbear.com/message/?prod=LoadComplete form and ask for it. Or download free trial of v.4.50 (https://smartbear.com/product/loadcomplete/free-tool/).
As for your question:
a) I tried both your regex and my version that uses backreferences ((rst_regardingobjectguid>)(.*)</\1) and all of them also worked for me when tested in the desktop regex application;
b) In order to check if your regex works in LoadUIWeb you may try this approach: https://support.smartbear.com/viewarticle/78669/#ID0E1E (though I am not sure if it existed in LoadUIWeb);
c) Finally, just to be on the safe side: you are writing that you are using regex and looking for the data in the request. Usually, data are searched for in the responces obtained from web server, extracted to the variables and the values of these variables are used to replace data in the subsequent requests been sent to the web server. Is this what you are trying to do? https://support.smartbear.com/viewarticle/78518/ help section may provide you with additional information.