Example from here on Meta.
Search for the term global
- results from various categories will appear, including “bug” and “discourse hub”.
Search for global category:bug
- relevant results are shown as expected.
Search for global category:discourse hub
- no results appear.
Search for global category:'discourse hub'
- no results appear.
Search for global category:"discourse hub"
- 11 results appear.
Search for global category:discourse_hub
- 11 results appear.
There is nothing in the search help to state double quotes or an underscore are required.