[Help-bash] Escaping in backquote string in double quoted strings -- query about bash intent

[KHMan]

Hi all,

Further to my posting on 2019-05-03, I would like to direct the 
following to those who work on bash. (Thanks to Greg and Andy for 
the replies, but it is not the answer that I was seeking since 
programmers will most certainly encounter these kind of things in 
an editor and then post bug tickets when highlighting breaks.)

If this is inappropriate for help-bash, I can repost it on the dev 
list. But here I am only trying to understand the _intention_ of 
the code so that I can code the appropriate highlighting fix -- 
it's really still a glorified shell user's query.


So looking at the source code for bash-5.0:

parse.y[4971]:
      /* Parse a matched pair of quote characters. */
      if MBTEST(shellquote (character))
        {
          push_delimiter (dstack, character);
	  ttok = parse_matched_pair (character, character, character, 
&ttoklen, (character == '`') ? P_COMMAND : 0);
          pop_delimiter (dstack);

I notice that many parse_matched_pair calls were bracketed by 
push_delimiter and pop_delimiter. But here:

parse.y[3734]:
      /* Parse an old-style command substitution within double 
quotes as a
         single word. */
      /* XXX - sh and ksh93 don't do this - XXX */
      else if MBTEST(open == '"' && ch == '`')
        {
          nestret = parse_matched_pair (0, '`', '`', &nestlen, rflags);

Here, for the case of a backquote string inside a double quoted 
string, there is no push_delimiter and pop_delimiter. If 
current_delimiter drives escape behaviour, this would account for 
the observed output for the test cases.

I guess the comment means something but I only know how to write 
simple bash scripts so I have no idea what it really means:
    Parse an old-style command substitution within
    double quotes as a single word.

So what's the intention of bash here? What is bash trying to do 
and is the escape behaviour for double quotes (\") in this 
situation (as presented in my first 2019-05-03 posting) unintended 
behaviour or intended behaviour? If it is intended behaviour, what 
is the rationale for it?


Thanks,
--
Cheers,
Kein-Hong Man (esq.)
Selangor, Malaysia