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Say which regexp ranges should be avoided

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* doc/lispref/searching.texi (Regexp Special): Say that
regular expressions like "[a-m-z]" and "[[:alpha:]-~]" should
be avoided, for the same reason that regular expressions like
"+" and "*" should be avoided: POSIX says their behavior is
undefined, and they are confusing anyway.  Also, explain
better what happens when the bound of a range is a raw 8-bit
byte; the old explanation appears to have been obsolete
anyway.  Finally, say that ranges like "[\u00FF-\xFF]" that
mix non-ASCII characters and raw 8-bit bytes should be
avoided, since it’s not clear what they should mean.
This commit is contained in:
Paul Eggert 2019-03-20 14:43:30 -07:00
parent 297a141ca3
commit 0924b27bca
1 changed files with 35 additions and 19 deletions
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doc/lispref/searching.texi
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@ -391,25 +391,18 @@ writing the starting and ending characters with a @samp{-} between them.
Thus, @samp{[a-z]} matches any lower-case @acronym{ASCII} letter.
Ranges may be intermixed freely with individual characters, as in
@samp{[a-z$%.]}, which matches any lower case @acronym{ASCII} letter
or @samp{$}, @samp{%} or period.
or @samp{$}, @samp{%} or period. However, the ending character of one
range should not be the starting point of another one; for example,
@samp{[a-m-z]} should be avoided.
If @code{case-fold-search} is non-@code{nil}, @samp{[a-z]} also
matches upper-case letters. Note that a range like @samp{[a-z]} is
not affected by the locale's collation sequence, it always represents
a sequence in @acronym{ASCII} order.
@c This wasn't obvious to me, since, e.g., the grep manual "Character
@c Classes and Bracket Expressions" specifically notes the opposite
@c behavior. But by experiment Emacs seems unaffected by LC_COLLATE
@c in this regard.
Note also that the usual regexp special characters are not special inside a
The usual regexp special characters are not special inside a
character alternative. A completely different set of characters is
special inside character alternatives: @samp{]}, @samp{-} and @samp{^}.
To include a @samp{]} in a character alternative, you must make it the
first character. For example, @samp{[]a]} matches @samp{]} or @samp{a}.
To include a @samp{-}, write @samp{-} as the first or last character of
the character alternative, or put it after a range. Thus, @samp{[]-]}
the character alternative, or as the upper bound of a range. Thus, @samp{[]-]}
matches both @samp{]} and @samp{-}. (As explained below, you cannot
use @samp{\]} to include a @samp{]} inside a character alternative,
since @samp{\} is not special there.)
@ -417,13 +410,34 @@ since @samp{\} is not special there.)
To include @samp{^} in a character alternative, put it anywhere but at
the beginning.
@c What if it starts with a multibyte and ends with a unibyte?
@c That doesn't seem to match anything...?
If a range starts with a unibyte character @var{c} and ends with a
multibyte character @var{c2}, the range is divided into two parts: one
spans the unibyte characters @samp{@var{c}..?\377}, the other the
multibyte characters @samp{@var{c1}..@var{c2}}, where @var{c1} is the
first character of the charset to which @var{c2} belongs.
The following aspects of ranges are specific to Emacs, in that POSIX
allows but does not require this behavior and programs other than
Emacs may behave differently:
@enumerate
@item
If @code{case-fold-search} is non-@code{nil}, @samp{[a-z]} also
matches upper-case letters.
@item
A range is not affected by the locale's collation sequence: it always
represents the set of characters with codepoints ranging between those
of its bounds, so that @samp{[a-z]} matches only ASCII letters, even
outside the C or POSIX locale.
@item
As a special case, if either bound of a range is a raw 8-bit byte, the
other bound should be a unibyte character, and the range matches only
unibyte characters.
@item
If the lower bound of a range is greater than its upper bound, the
range is empty and represents no characters. Thus, @samp{[b-a]}
always fails to match, and @samp{[^b-a]} matches any character,
including newline. However, the lower bound should be at most one
greater than the upper bound; for example, @samp{[c-a]} should be
avoided.
@end enumerate
A character alternative can also specify named character classes
(@pxref{Char Classes}). This is a POSIX feature. For example,
@ -431,6 +445,8 @@ A character alternative can also specify named character classes
Using a character class is equivalent to mentioning each of the
characters in that class; but the latter is not feasible in practice,
since some classes include thousands of different characters.
A character class should not appear as the lower or upper bound
of a range.
@item @samp{[^ @dots{} ]}
@cindex @samp{^} in regexp