Raku
What is an 'atom'?
The docs for <ws> say the token is autoplaced after an "atom", but what exactly is an atom in a grammar doesn't seem to be explained anywhere.
Any part of a regex that you can identify as having any meaning is an atom.
So for example in \d ** 4, the \d and 4 are terms, then ** is an operator, and all three of those are atoms.
The statement that <.ws> is automatically inserted after an atom isn't quite always true. In the example above, that would lead to \d <.ws> ** <.ws> 4, which makes no sense. A more precise wording would be that <.ws> is placed after terms and closing parenthesis/brackets, and that operators typically special-case <.ws> handling.
I'm reopening this, because the concept of atom in a regex probably deserves its own fleshed out subsection somewhere. The answer to this question on SO fully relied on the clarification of this concept. Additionally, non-capturing grouping also creates an atom, so it should be clarified that happens.
