Re: Unquoted special characters in regexps

[martin rudalics <rudalics@gmx.at>]

 > Martin Rudalics wrote:
 >
 >    `]' is _also_ special in a character alternative, like `^'.  `-' is
 >    special _only_ in a character alternative.
 >
 > I may be overlooking something, but _which_ special meaning does `]'
 > have outside of character alternatives?

That of closing a character alternative.  When you write

(defvar foo "]")
(defvar bar "\\]")

you can't interchangeably use `foo' and `bar' in an arbitrary regular
expression.  Some people call this "referential transparency".

 > You are using \\] to quote `]'.  Could that possibly clear up any
 > confusion or does it just add confusion?  I personally believe the
 > latter.  Is there a situation where \\ can be used to prevent `]' from
 > having a special meaning?  In "[a\\]b]", the first `]' still has a
 > special meaning, even though there might be some optical illusion
 > making it look "quoted", the second `]' has no special meaning.

According to the Elisp manual "[a\\]b]" is poor practice.  You probably
mean "[a\\]b\\]" here.  Anyway, the first `]' has a special meaning but
it's not "inside" the character alternative.  It does have a special
meaning because `]' is special _outside_ character alternatives.

 > I believe that a `]' should not be quoted at all if it is outside a
 > character alternative, where it has no special meaning, unless I am
 > overlooking something.  (Just tell what, in that case.)

That of terminating a character alternative.

 >
 > ELISP> (string-match "[a\\]b]" "]")
 > nil
 > ELISP> (string-match "[a\\]b]" "\\b")
 > nil
 > ELISP> (string-match "[a\\]b]" "\\b]")
 > 0

According to the Elisp manual all these exhibit "poor practice" since
you didn't quote the second `]'s.  You should have complained about that
when you read the manual.