This section describes some variables that hold regular expressions used for certain purposes in editing:
This is the regular expression describing line-beginnings that separate
pages. The default value is "^\014" (i.e., "^^L" or
"^\C-l"); this matches a line that starts with a formfeed
character.
The following two regular expressions should not assume the match always starts at the beginning of a line; they should not use ‘^’ to anchor the match. Most often, the paragraph commands do check for a match only at the beginning of a line, which means that ‘^’ would be superfluous. When there is a nonzero left margin, they accept matches that start after the left margin. In that case, a ‘^’ would be incorrect. However, a ‘^’ is harmless in modes where a left margin is never used.
This is the regular expression for recognizing the beginning of a line
that separates paragraphs. (If you change this, you may have to
change paragraph-start also.) The default value is
"[ \t\f]*$", which matches a line that consists entirely of
spaces, tabs, and form feeds (after its left margin).
This is the regular expression for recognizing the beginning of a line
that starts or separates paragraphs. The default value is
"\f\\|[ \t]*$", which matches a line containing only
whitespace or starting with a form feed (after its left margin).
If non-nil, the value should be a regular expression describing
the end of a sentence, including the whitespace following the
sentence. (All paragraph boundaries also end sentences, regardless.)
If the value is nil, as it is by default, then the function
sentence-end constructs the regexp. That is why you
should always call the function sentence-end to obtain the
regexp to be used to recognize the end of a sentence.
This function returns the value of the variable sentence-end,
if non-nil. Otherwise it returns a default value based on the
values of the variables sentence-end-double-space
(see Definition of sentence-end-double-space),
sentence-end-without-period, and
sentence-end-without-space.