I was playing around with the rx function for generating regular expressions from sexps in ELISP but couldn't figure out how to generate the regular expression "\\documentclass" for use in org-export-latex-classes:
(rx "\\documentclass")
(rx "\\" "documentclass")
(rx (char "\\") "documentclass")
Which when evaluated give respectively the following outputs:
"\\\\documentclass"
"\\\\documentclass"
"[\\]documentclass"
Is "\\documentclass" equivalent to "[\\]documentclass"?---I think it is, but am not sure. Can I generate the former using rx?
Edit: Whilst the question was valid I realize my motivation was not; because org-export-latex-classes use strings not regular expressions.
Emacs requires \ to be escaped in the double-quoted read syntax for strings and so, when the code is processed by the lisp reader, "\\" evaluates to a string object containing a single \ character; and hence that single backslash is what the regexp engine sees when it uses that string object.
However, a \ in a regexp also has an escaping function, which means that the sequence \\ in a regexp matches a single \.
To represent the sequence \\ in (the read syntax for) Emacs strings, each of those backslashes must itself be escaped by prefixing it with a backslash.
Therefore "\\\\" evaluates to a string containing \\ which can be used as a regexp matching a single \.
Within a regexp character alternative sequence, however, backslashes are literal (they do not escape the following character); and so [\], represented by the string "[\\]", matches a single backslash -- the only possible match for that single-character set.
So used as regexps, the strings "\\\\" and "[\\]" match the same thing.
The string "\\documentclass" as a regexp is effectively the same as "documentclass" with no backslashes at all, as it is the d which is being escaped in the regexp (which is valid, but of course unnecessary).
The elisp manual explains this as follows:
`\' has two functions: it quotes the special characters (including
`\'), and it introduces additional special constructs.
Because `\' quotes special characters, `\$' is a regular
expression that matches only `$', and `\[' is a regular expression
that matches only `[', and so on.
Note that `\' also has special meaning in the read syntax of Lisp
strings (*note String Type::), and must be quoted with `\'. For
example, the regular expression that matches the `\' character is
`\\'. To write a Lisp string that contains the characters `\\',
Lisp syntax requires you to quote each `\' with another `\'.
Therefore, the read syntax for a regular expression matching `\'
is `"\\\\"'.
[...]
As a `\' is not special inside a character alternative, it can never
remove the special meaning of `-' or `]'. So you should not quote
these characters when they have no special meaning either. This would
not clarify anything, since backslashes can legitimately precede these
characters where they _have_ special meaning, as in `[^\]' (`"[^\\]"'
for Lisp string syntax), which matches any single character except a
backslash.
C-hig (elisp) Regexp Special RET