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?
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 (
You already have the answer, but a built-in aide for creating regular expressions inside Emacs is re-builder.
M-x re-builder
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.
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)
来源：https://stackoverflow.com/questions/538842/emacs-regular-expressions-in-lisp-need-to-be-double-escaped-why