A list of functions, regex, commands from the drop-down menu in the search bar could help.
There is something similar for example in AIMP player settings:
![Image](https://i.imgur.com/e4XIFgH.png)
All could be grouped in categories and with simple description. After hovering over a given item from the list, a balloon with a more detailed description could appear. Each selected item would be inserted at the cursor position. Clicking LMB twice at inserted/typed function is selecting it, but with this new improvement it could paste/change it to selected from the functions menu.
In the menu, of course, there should be standard system commands such as Copy, Paste etc.
In addition, to make it easier when using keyboard, when you start to type a function, a list of matching functions could be displayed below, which could be selected by pressing the down/up/enter keys.