So, your goal is to search and replace?
According to Visual Studio Code’s keyboard shortcuts PDF, you can press Ctrl + H on Windows and Linux, or ⌥⌘F on Mac to enable the search and replace tool:

If you mean to disable the code, you just have to put <h1> in search, and replace to ####.
But if you want to use this regex instead, you may enable it in the icon:
and use the regex: <h1>(.+?)<\/h1> and replace to: #### $1.
And as @tpartee suggested, here is some more information about Visual Studio’s engine if you would like to learn more:
- Find and Replace Window (documentation)
- Quick Replace, Find and Replace Window (documentation)
- What flavor of Regex does Visual Studio Code use?