RE: About regexp

["Drew Adams" <drew.adams@oracle.com>]

> The default value of the variable `sentence-end-base' is:
> [.?!][]\"'")}]*
> There is a backslash in the second part of the regexp, why?

C-h v sentence-end-base tells you:

 sentence-end-base is a variable defined in `paragraphs.el'.
 Its value is "[.?!][]\"'")}]*" 

IOW, the value is a string.  The double-quote chars shown surrounding the string
are not characters in the string.  The double-quote char shown just before the
single-quote char, ', is in the string.  When printed, that char is escaped with
a backslash.  The char in the string following the ' char is a different char (a
curly quote) - it is not used to indicate the string limits, so it need not be
escaped to distinguish it.