emacs - regular expressions in lisp need to be double-escaped...why?

Question

I've been playing around with emacs lisp, and I wanted to write a little function to do a regular expression search and replace. I had a heck of a time getting the regular expression to work correctly because I didn't realize that all the special characters need to be double escaped when writing lisp code (but not when using query-replace-regexp interactively!).

So for example, using query-replace-regexp interactively you can use

^\(.*\)[\t]-.*$

but when writing elisp code you need to double escape everything like so:

^\\(.*\\)[\t]-.*$

I finally found a reference to this in a Steve Yegge article, but I was wondering if anyone knew why this is?

Answer 1

It's because you need to escape backslashes in strings. If you don't escape the backslash of \( in the string it turns out to be just (

Answer 2

scottfrazier is correct, one escape is parsed when the string is read, another is parsed when creating the regular expression. It's fairly easy to remember, but it can become a pain, especially when you're trying to match a literal backslash '\'. You end up having to do it four times '\\\\' because you have to double-slash to match the slash in both the initial string parse and the regular expression parse.

And when you write on Stack Overflow about this problem you have to use 8 slashes because markdown uses the slash for an escape character as well.

Answer 3

You already have the answer, but a built-in aide for creating regular expressions inside Emacs is re-builder.

M-x re-builder

Answer 4

FWIW, emacs-lisp-mode will fontify the special expressions (like "\\(" and "\\)" for you. You can then change the faces to be something that stands out. (They are font-lock-regexp-grouping-construct and font-lock-regexp-grouping-backslash)