RE: regexp to match a sexp?

[Drew Adams]

    > I'm looking for a regexp that will match (only) a sexp.

    In general, it's not possible.  Regexps cannot match recursive grammars.

I know that. Again, as I said:

    > I'm less interested in hearing "it can't be done" than in
    > attempts to do the job, even if in a rough way.

    Are you excluding sublists?

No, as I said. Excluding them would be one legitimate approximation. But the
real exercise is to allow for some degree of nesting.

    With sublists excluded, it's possible,
    but note that \s(.*\s) you cannot ensure that the opening bracket is
    the corresponding character to the closing bracket.

Yes.

Of course, with no sublists, you can use \s(\S)*\s).

    You'll have to write:

    (ELEMENTS)\|\[ELEMENTS\]\|{ELEMENTS}\|<ELEMENTS>\|...
    ELEMENTS ::= "\([^"\\]*\|\\.\)*"\|[^"]*

I'm interested in candidate regexps that might be useful in limited
contexts. Yes, different regexps will have different limitations, and
therefore be differently useful.

Think of this as a quick-and-dirty `C-M-s' that might (*might*) just
usefully find a sexp some of the time.

    > 2. Strings and the possible escaping of `"' would be one
    > headache that would need to be dealt with carefully, as always.

    No, it's simplistic to deal with them in regexps. See above.

In practice, we do deal with them in Emacs, albeit with limited success. I'm
not trying to write a sexp grammar; I'm asking about what can be done,
practically, with regexps, in terms of matching sexps.

    > 3. It would need to be effectively recursive or some
    > approximation thereof, for example, with some limit placed
    > on nesting. That is, it would need to allow for nested sexps.

    Yes, that's why it's not possible with regexps. Regexps are not
    recursive by definition.

Yes, I know that.

Some approximations can be made. You proposed an approximation of zero
nesting. That in itself can be useful in some contexts, but one level of
nesting is also useful, and two levels,...

    > 4. It would need to deal properly with quoting, `''. Dealing
    > with backquote syntax, ``', would be a plus.
    >
    > Can something like this be done in a reasonable way? What's a
    > good regexp that you could use, e.g., to search for one or
    > more sexps?
    >
    > I'm not looking for a way to search for or scan a sexp
    > *without* using a regexp; I know there are ways to do that.
    > I'm wondering what can be done *with* a regexp. IOW, imagine
    > that all you have is `C-M-s' (but don't worry
    > about the expression being too complex to type interactively).
    >
    > I'm less interested in hearing "it can't be done" than in
    > attempts to do the job, even if in a rough way.

    Well, since it's not possible, ita can't be done, but you can still go
    thru the mirror and see if it's possible, since you prefer
    that.  Good bye.

    Now, if your purpose is to _*PARSE*_ sexps  instead of using regexps,

I stated that my purpose was the opposite. If I ask about turtles in
Alabama, why do you tell me what you know about wine in Tuscany? ;-)

    then you can easily write a sexp parser.  This is one of the simpliest
    grammar there is.  In emacs, of course you can use the provided sexp
    parser, with functions such as: forward-sexp, backward-sexp,
    (thing-at-point 'sexp), read-from-string, etc...

I know about those. No, I'm not interested in them here.

I'm interested in how much that's interesting could be done with a regexp.
The question is what you can do with `C-M-s' - what interesting regexps
would you use to find which classes of sexp or almost-sexp?