[RFC] Alternative to sub/superscript regexp

[Nicolas Goaziou <n.goaziou@gmail.com>]

Hello,

For the record `org-match-substring-regexp' is a variation on:

"\\(\\S-\\)\\([_^]\\)\\(\\(?:\\*\\|[-+]?[^-+*!@#$%^_ \t\r\n,:\"?<>~;./{}=()]+\\)\\)\\)"

I think it is a bit convoluted and therefore difficult to predict. For
example, as recent bug report showed, you may tend to interpret
a_b[fn:1] as

   a_{b}[fn:1]

but, in fact, it is equivalent to

   a_{b[fn}:1]

Of course, we can prevent this by forbidding "[" and "]" in the last
part of the regexp. But I wonder if there's something better to do.

The idea behind this regexp is that we should be able to write simple
sub/superscript, including numbers and entities, without requiring curly
braces (see `org-use-sub-superscripts' docstring for details). Maybe
something like the following could be an interesting alternative:

  "\\(\\S-\\)\\([_^]\\)\\(\\*\\|[+-]?\\(?:\\w\\|[0-9.,\\]\\)*\\(\\w\\|[0-9]\\)\\)"

That is, without braces, either an asterisk or any combination of word,
number, dot, comma and backslash characters, which may start with either
a plus or a minus sign but cannot end with either a dot or a comma.

I find it arguably more predictable (no inverted class). Also, we "gain"
the following:

  a^3.14. <=> a^{3.14}.

At the moment, a^3.14. <=> a^{3}.14.

What do you think?


Regards,

-- 
Nicolas Goaziou