I added an example for how to make find-in-file start with regexp search toggled on to our repository of examples. It was a redditor who needed it: https://www.reddit.com/r/vscode/comments/v456kj/comment/ibcrw5s/?utm_source=share&utm_medium=web2x&context=3
This SO question as well, please upvote: https://stackoverflow.com/questions/57527931/vscode-how-to-enable-regex-find-by-command-rather-than-toggle/72520482#72520482