Re: Regular expressions and user-escaped characters

[Stefan Monnier via Users list for the GNU Emacs text editor <help-gnu-emacs@gnu.org>]

> Hi, what do you do in a regular expression if you want to match a character,
> but not a the same character that has been escaped by the user. E.g., if
> I want my regular expression to look for ?\[ (ASCII 91), matching string "["
> and "a[a" but not string "\\[" or "a\\[a", if you follow me. Is this
> possible with just a regular expression?

The "usual" way we do that is with the godawful:

    "\\(?:^\\|[^\\]\\(?:\\\\\\\\\\)*\\)\\["

This is careful to match the [ if it's preceded by an even number
of backslashes.  But beware that it makes more than the actual [, so if
you start the search from a point that's looking at a [, it won't find
it (except if it's at the beginning of the line).

> If not, what is a good workaround?

Just use a regexp which matches all [ (regardless of any previous
backslashes) and then check afterwards, in ELisp, whether it's preceded
by an odd number of backslashes, e.g. with something like

    (save-excursion
      (goto-char <FOO>)
      (zerop (% (skip-chars-backward "\\") 2)))


- Stefan