rx-to-string and no-group argument

Question

Test

(dolist (form (list '(+ "a")
                    '(+ "ab")
                    '(or "aa" "bb")
                    '(: bow "a" eow)))
  (dolist (no-group (list nil t))
    (princ (rx-to-string form no-group))
    (terpri))
  (terpri))

Output

\(?:a+\)
a+

\(?:\(?:ab\)+\)
\(?:ab\)+

\(?:\(?:aa\|bb\)\)
\(?:aa\|bb\)

\(?:\<a\>\)
\<a\>

Am I right in assuming that the value of NO-GROUP argument in rx-to-string makes no difference in how the returned regexp string will behave and that it is only for cosmetic reasons?

If I go through a package's code and changes NO-GROUP from nil to t or vice versa in every occurrence of rx-to-string, is it safe to assume that nothing will break?

Answer 1

No, you cannot assume that nothing will break.

The groups are there not just for cosmetic reasons. These are shy groups:

A shy group serves the first two purposes of an ordinary group (controlling the nesting of other operators), but it does not get a number, so you cannot refer back to its value with ‘\digit’. Shy groups are particularly useful for mechanically-constructed regular expressions, because they can be added automatically without altering the numbering of ordinary, non-shy groups.

This means that the regexps themselves are equivalent, but their combinations are not.