Re: Regular expressions and user-escaped characters

[Joost Kremers <joostkremers@fastmail.fm>]

On Mon, Dec 02 2024, Christopher Howard wrote:
> 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?

You may get away with something like "[^\\][[]", though keep in mind that
that does not match a ?[ not preceded by a backslash, but rather a ?[
preceded by a character that is not a backslash. Depending on your use
case, that might suffice, though, esp. if you use a capturing group:

```
(let ((str "a[a"))
  (when (string-match "[^\\]\\([[]\\)" str)
    (match-string 1 str)))

=> "["
```

vs.:

```
(let ((str "a\\[a"))
  (when (string-match "[^\\]\\([[]\\)" str)
    (match-string 1 str)))
=> nil
```

The "proper" way to do this would be to use negative lookbehind,
`"(?<!\\)[[])"`, but Emacs' regexp engine does not support that.

> If not, what is a good workaround? I was wondering about, say, replacing
> all the escaped characters first with some uncommon character (like a
> control code) and then converting back afterwards. But then I suppose I
> would need to do a check for that uncommon character first.

That would probably work.

-- 
Joost Kremers
Life has its moments