Hi, is .jpg part of the extension? In the Renaming menu File/Folder Extensions > Rename File Extensions as Being Part of the File Name With this option selected, the file name extensions will be ignored and your renaming operations will always affect the entire file name. Use this option with great care, as Windows relies on the file name extension to decide what to do with a file.
After putting the checkmark inside for "Renaming Options, File/Folder Extensions, Rename File Extensions". Then RegEx(1) could probably just use a "Match" and "Replace" something like... (.+\.jpg) \1
But with any names like start.jpg-middle-end.mp4, its much safer with something like... (.+\.jpg)[^.]+$ \1