Maybe not an issue in your case but your regexp search might match on tags you don’t want them to like “Docs”, “Documents”, “Doctor”, “Domains”, etc.
@TW_Tones’ solution would be less error-prone, I think, and would be equivalent to [tag[Domain]] :or[tag[Doc]]
which treats the specified filter operand (“Domain Doc”) as “a list of tokens separated by whitespace, and matches if at least one of the tokens appear in the string”. Note that it’s not clear to me whether internally this is more efficient than using regexp, but it does offer another way to achieve your desired result. You might want to do some testing with “big data” (i.e., lots of tiddlers with lots of tags) to see if it gives better performance than the regexp search mode flag.