I was able to build this HUGE expression in the last one or two months with the help of an automatic expression builder functionality.
This feature made possible to TEST any input strings and AUTOMATICALLY GENERATE the corresponding expression quickly. It is not doable by hand.
I am not able to locate this functionality anymore, even though I have the Experimental Feature's setting enabled. This feature is imperative for me to continue working with Power Automate. How do I get it back?