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More regexp advice and clarifications

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* doc/lispref/searching.texi (Regexp Special): Simplify style
advice for order of ], ^, and - in character alternatives.
Stick with saying that it’s not a good idea to put ‘-’ after a
range.  Remove the special case about raw 8-bit bytes and
unibyte characters, as this documentation is confusing and
seems to be incorrect in some cases.  Say that z-a is the
preferred style for reversed ranges, since it’s clearer and is
typically what’s used in practice.  Mention some bad styles:
duplicates in character alternatives, ranges that denote <=3
characters, and ‘-’ as the first character.
This commit is contained in:
Paul Eggert 2019-04-02 00:17:37 -07:00
parent f81ec28f4f
commit 076ed98ff6
1 changed files with 31 additions and 21 deletions
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doc/lispref/searching.texi
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@ -398,17 +398,11 @@ range should not be the starting point of another one; for example,
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 last character of the character alternative,
tho you can also put it first or after 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.)
To include @samp{^} in a character alternative, put it anywhere but at
the beginning.
To include @samp{]} in a character alternative, put it at the
beginning. To include @samp{^}, put it anywhere but at the beginning.
To include @samp{-}, put it at the end. Thus, @samp{[]^-]} matches
all three of these special characters. You cannot use @samp{\} to
escape these three characters, since @samp{\} is not special here.
The following aspects of ranges are specific to Emacs, in that POSIX
allows but does not require this behavior and programs other than
@ -426,17 +420,33 @@ 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.
If the lower bound of a range is greater than its upper bound, the
range is empty and represents no characters. Thus, @samp{[z-a]}
always fails to match, and @samp{[^z-a]} matches any character,
including newline. However, a reversed range should always be from
the letter @samp{z} to the letter @samp{a} to make it clear that it is
not a typo; for example, @samp{[+-*/]} should be avoided, because it
matches only @samp{/} rather than the likely-intended four characters.
@end enumerate
Some kinds of character alternatives are not the best style even
though they are standardized by POSIX and are portable. They include:
@enumerate
@item
A character alternative can include duplicates. For example,
@samp{[XYa-yYb-zX]} is less clear than @samp{[XYa-z]}.
@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.
A range can denote just one, two, or three characters. For example,
@samp{[(-(]} is less clear than @samp{[(]}, @samp{[*-+]} is less clear
than @samp{[*+]}, and @samp{[*-,]} is less clear than @samp{[*+,]}.
@item
A @samp{-} also appear at the beginning of a character alternative, or
as the upper bound of a range. For example, although @samp{[-a-z]} is
valid, @samp{[a-z-]} is better style; and although @samp{[!--/]} is
valid, @samp{[!-,/-]} is clearer.
@end enumerate
A character alternative can also specify named character classes
@ -452,7 +462,7 @@ of a range.
@cindex @samp{^} in regexp
@samp{[^} begins a @dfn{complemented character alternative}. This
matches any character except the ones specified. Thus,
@samp{[^a-z0-9A-Z]} matches all characters @emph{except} letters and
@samp{[^a-z0-9A-Z]} matches all characters @emph{except} ASCII letters and
digits.
@samp{^} is not special in a character alternative unless it is the first