Return the regexp describing the end of a sentence.
This function returns either the value of the variable `sentence-end'
if it is non-nil, or the default value constructed from the
variables `sentence-end-base', `sentence-end-double-space',
`sentence-end-without-period' and `sentence-end-without-space'.
The default value specifies that in order to be recognized as the
end of a sentence, the ending period, question mark, or exclamation point
must be followed by two spaces, with perhaps some closing delimiters
in between. See Info node `(elisp)Standard Regexps'.
Regexp describing the end of a sentence.
The value includes the whitespace following the sentence.
All paragraph boundaries also end sentences, regardless.
The value nil means to use the default value defined by the
function `sentence-end'. You should always use this function
to obtain the value of this variable.