Call them “bracket expressions” more consistently
Emacs comments and doc were inconsistent about the name used for regexps like [a-z]. Sometimes it called them “character alternatives”, sometimes “character sets”, sometimes “bracket expressions”. Prefer “bracket expressions” as it is less confusing: POSIX and most other programs’ doc uses “bracket expressions”, “alternative” is also used in the Emacs documentation to talk about ...\|... in regexps, and “character set” normally has a different meaning in Emacs.
This commit is contained in:
Paul Eggert
parent
5dfe3f21d1
commit
94d8eeeff4
4 changed files with 45 additions and 45 deletions
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@ -950,8 +950,8 @@ features used mainly in Lisp programs.
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@dfn{special constructs} and the rest are @dfn{ordinary}. An ordinary
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character matches that same character and nothing else. The special
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characters are @samp{$^.*+?[\}. The character @samp{]} is special if
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it ends a character alternative (see below). The character @samp{-}
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is special inside a character alternative. Any other character
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it ends a bracket expression (see below). The character @samp{-}
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is special inside a bracket expression. Any other character
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appearing in a regular expression is ordinary, unless a @samp{\}
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precedes it. (When you use regular expressions in a Lisp program,
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each @samp{\} must be doubled, see the example near the end of this
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@ -1033,11 +1033,11 @@ you search for @samp{a.*?$} against the text @samp{abbab} followed by
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a newline, it matches the whole string. Since it @emph{can} match
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starting at the first @samp{a}, it does.
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@cindex bracket expression
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@cindex set of alternative characters, in regular expressions
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@cindex character set, in regular expressions
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@item @kbd{[ @dots{} ]}
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is a @dfn{set of alternative characters}, or a @dfn{character set},
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beginning with @samp{[} and terminated by @samp{]}.
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is a @dfn{bracket expression}, which matches one of a set of characters.
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In the simplest case, the characters between the two brackets are what
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this set can match. Thus, @samp{[ad]} matches either one @samp{a} or
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@ -1057,7 +1057,7 @@ Greek letters.
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@cindex character classes, in regular expressions
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You can also include certain special @dfn{character classes} in a
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character set. A @samp{[:} and balancing @samp{:]} enclose a
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character class inside a set of alternative characters. For instance,
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character class inside a bracket expression. For instance,
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@samp{[[:alnum:]]} matches any letter or digit. @xref{Char Classes,,,
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elisp, The Emacs Lisp Reference Manual}, for a list of character
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classes.
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@ -1125,7 +1125,7 @@ no preceding expression on which the @samp{*} can act. It is poor practice
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to depend on this behavior; it is better to quote the special character anyway,
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regardless of where it appears.
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As a @samp{\} is not special inside a set of alternative characters, it can
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As a @samp{\} is not special inside a bracket expression, it can
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never remove the special meaning of @samp{-}, @samp{^} or @samp{]}.
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You should not quote these characters when they have no special
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meaning. This would not clarify anything, since backslashes
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@ -278,10 +278,10 @@ character is a simple regular expression that matches that character
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and nothing else. The special characters are @samp{.}, @samp{*},
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@samp{+}, @samp{?}, @samp{[}, @samp{^}, @samp{$}, and @samp{\}; no new
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special characters will be defined in the future. The character
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@samp{]} is special if it ends a character alternative (see later).
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The character @samp{-} is special inside a character alternative. A
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@samp{]} is special if it ends a bracket expression (see later).
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The character @samp{-} is special inside a bracket expression. A
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@samp{[:} and balancing @samp{:]} enclose a character class inside a
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character alternative. Any other character appearing in a regular
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bracket expression. Any other character appearing in a regular
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expression is ordinary, unless a @samp{\} precedes it.
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For example, @samp{f} is not a special character, so it is ordinary, and
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@ -374,19 +374,19 @@ expression @samp{c[ad]*?a}, applied to that same string, matches just
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permits the whole expression to match is @samp{d}.)
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@item @samp{[ @dots{} ]}
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@cindex character alternative (in regexp)
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@cindex bracket expression (in regexp)
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@cindex @samp{[} in regexp
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@cindex @samp{]} in regexp
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is a @dfn{character alternative}, which begins with @samp{[} and is
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is a @dfn{bracket expression}, which begins with @samp{[} and is
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terminated by @samp{]}. In the simplest case, the characters between
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the two brackets are what this character alternative can match.
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the two brackets are what this bracket expression can match.
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Thus, @samp{[ad]} matches either one @samp{a} or one @samp{d}, and
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@samp{[ad]*} matches any string composed of just @samp{a}s and @samp{d}s
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(including the empty string). It follows that @samp{c[ad]*r}
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matches @samp{cr}, @samp{car}, @samp{cdr}, @samp{caddaar}, etc.
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You can also include character ranges in a character alternative, by
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You can also include character ranges in a bracket expression, by
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writing the starting and ending characters with a @samp{-} between them.
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Thus, @samp{[a-z]} matches any lower-case @acronym{ASCII} letter.
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Ranges may be intermixed freely with individual characters, as in
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@ -395,7 +395,7 @@ or @samp{$}, @samp{%} or period. However, the ending character of one
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range should not be the starting point of another one; for example,
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@samp{[a-m-z]} should be avoided.
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A character alternative can also specify named character classes
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A bracket expression can also specify named character classes
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(@pxref{Char Classes}). For example, @samp{[[:ascii:]]} matches any
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@acronym{ASCII} character. Using a character class is equivalent to
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mentioning each of the characters in that class; but the latter is not
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@ -404,9 +404,9 @@ different characters. A character class should not appear as the
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lower or upper bound of a range.
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The usual regexp special characters are not special inside a
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character alternative. A completely different set of characters is
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bracket expression. A completely different set of characters is
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special: @samp{]}, @samp{-} and @samp{^}.
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To include @samp{]} in a character alternative, put it at the
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To include @samp{]} in a bracket expression, put it at the
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beginning. To include @samp{^}, put it anywhere but at the beginning.
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To include @samp{-}, put it at the end. Thus, @samp{[]^-]} matches
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all three of these special characters. You cannot use @samp{\} to
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@ -444,7 +444,7 @@ characters and raw 8-bit bytes, but not non-ASCII characters. This
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feature is intended for searching text in unibyte buffers and strings.
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@end enumerate
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Some kinds of character alternatives are not the best style even
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Some kinds of bracket expressions are not the best style even
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though they have a well-defined meaning in Emacs. They include:
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@enumerate
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@ -458,7 +458,7 @@ Unicode character escapes can help here; for example, for most programmers
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@samp{[ก-ฺ฿-๛]} is less clear than @samp{[\u0E01-\u0E3A\u0E3F-\u0E5B]}.
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@item
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Although a character alternative can include duplicates, it is better
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Although a bracket expression can include duplicates, it is better
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style to avoid them. For example, @samp{[XYa-yYb-zX]} is less clear
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than @samp{[XYa-z]}.
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@ -469,30 +469,30 @@ is simpler to list the characters. For example,
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than @samp{[ij]}, and @samp{[i-k]} is less clear than @samp{[ijk]}.
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@item
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Although a @samp{-} can appear at the beginning of a character
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alternative or as the upper bound of a range, it is better style to
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put @samp{-} by itself at the end of a character alternative. For
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Although a @samp{-} can appear at the beginning of a bracket
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expression or as the upper bound of a range, it is better style to
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put @samp{-} by itself at the end of a bracket expression. For
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example, although @samp{[-a-z]} is valid, @samp{[a-z-]} is better
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style; and although @samp{[*--]} is valid, @samp{[*+,-]} is clearer.
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@end enumerate
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@item @samp{[^ @dots{} ]}
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@cindex @samp{^} in regexp
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@samp{[^} begins a @dfn{complemented character alternative}. This
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@samp{[^} begins a @dfn{complemented bracket expression}. This
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matches any character except the ones specified. Thus,
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@samp{[^a-z0-9A-Z]} matches all characters @emph{except} ASCII letters and
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digits.
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@samp{^} is not special in a character alternative unless it is the first
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@samp{^} is not special in a bracket expression unless it is the first
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character. The character following the @samp{^} is treated as if it
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were first (in other words, @samp{-} and @samp{]} are not special there).
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A complemented character alternative can match a newline, unless newline is
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A complemented bracket expression can match a newline, unless newline is
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mentioned as one of the characters not to match. This is in contrast to
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the handling of regexps in programs such as @code{grep}.
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You can specify named character classes, just like in character
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alternatives. For instance, @samp{[^[:ascii:]]} matches any
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You can specify named character classes, just like in bracket
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expressions. For instance, @samp{[^[:ascii:]]} matches any
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non-@acronym{ASCII} character. @xref{Char Classes}.
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@item @samp{^}
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@ -556,7 +556,7 @@ that matches only empty strings, as Emacs has bugs in this area.
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For example, it is unwise to use @samp{\b*}, which can be omitted
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without changing the documented meaning of the regular expression.
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As a @samp{\} is not special inside a character alternative, it can
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As a @samp{\} is not special inside a bracket expression, it can
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never remove the special meaning of @samp{-}, @samp{^} or @samp{]}.
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You should not quote these characters when they have no special
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meaning. This would not clarify anything, since backslashes
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@ -565,23 +565,23 @@ special meaning, as in @samp{[^\]} (@code{"[^\\]"} for Lisp string
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syntax), which matches any single character except a backslash.
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In practice, most @samp{]} that occur in regular expressions close a
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character alternative and hence are special. However, occasionally a
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bracket expression and hence are special. However, occasionally a
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regular expression may try to match a complex pattern of literal
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@samp{[} and @samp{]}. In such situations, it sometimes may be
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necessary to carefully parse the regexp from the start to determine
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which square brackets enclose a character alternative. For example,
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@samp{[^][]]} consists of the complemented character alternative
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which square brackets enclose a bracket expression. For example,
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@samp{[^][]]} consists of the complemented bracket expression
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@samp{[^][]} (which matches any single character that is not a square
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bracket), followed by a literal @samp{]}.
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The exact rules are that at the beginning of a regexp, @samp{[} is
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special and @samp{]} not. This lasts until the first unquoted
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@samp{[}, after which we are in a character alternative; @samp{[} is
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@samp{[}, after which we are in a bracket expression; @samp{[} is
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no longer special (except when it starts a character class) but @samp{]}
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is special, unless it immediately follows the special @samp{[} or that
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@samp{[} followed by a @samp{^}. This lasts until the next special
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@samp{]} that does not end a character class. This ends the character
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alternative and restores the ordinary syntax of regular expressions;
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@samp{]} that does not end a character class. This ends the bracket
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expression and restores the ordinary syntax of regular expressions;
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an unquoted @samp{[} is special again and a @samp{]} not.
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@node Char Classes
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@ -592,8 +592,8 @@ an unquoted @samp{[} is special again and a @samp{]} not.
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@cindex alpha character class, regexp
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@cindex xdigit character class, regexp
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Below is a table of the classes you can use in a character
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alternative, and what they mean. Note that the @samp{[} and @samp{]}
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Below is a table of the classes you can use in a bracket
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expression, and what they mean. Note that the @samp{[} and @samp{]}
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characters that enclose the class name are part of the name, so a
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regular expression using these classes needs one more pair of
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brackets. For example, a regular expression matching a sequence of
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@ -920,7 +920,7 @@ with a symbol-constituent character.
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@kindex invalid-regexp
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Not every string is a valid regular expression. For example, a string
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that ends inside a character alternative without a terminating @samp{]}
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that ends inside a bracket expression without a terminating @samp{]}
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is invalid, and so is a string that ends with a single @samp{\}. If
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an invalid regular expression is passed to any of the search functions,
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an @code{invalid-regexp} error is signaled.
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@ -957,7 +957,7 @@ deciphered as follows:
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@table @code
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@item [.?!]
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The first part of the pattern is a character alternative that matches
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The first part of the pattern is a bracket expression that matches
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any one of three characters: period, question mark, and exclamation
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mark. The match must begin with one of these three characters. (This
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is one point where the new default regexp used by Emacs differs from
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@ -969,7 +969,7 @@ The second part of the pattern matches any closing braces and quotation
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marks, zero or more of them, that may follow the period, question mark
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or exclamation mark. The @code{\"} is Lisp syntax for a double-quote in
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a string. The @samp{*} at the end indicates that the immediately
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preceding regular expression (a character alternative, in this case) may be
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preceding regular expression (a bracket expression, in this case) may be
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repeated zero or more times.
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@item \\($\\|@ $\\|\t\\|@ @ \\)
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@ -1920,7 +1920,7 @@ attempts. Other zero-width assertions may also bring benefits by
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causing a match to fail early.
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@item
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Avoid or-patterns in favor of character alternatives: write
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Avoid or-patterns in favor of bracket expressions: write
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@samp{[ab]} instead of @samp{a\|b}. Recall that @samp{\s-} and @samp{\sw}
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are equivalent to @samp{[[:space:]]} and @samp{[[:word:]]}, respectively.
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@ -3012,7 +3012,7 @@ but does not support all the Emacs escapes.
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@item
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In POSIX BREs, it is an implementation option whether @samp{^} is special
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after @samp{\(}; GNU @command{grep} treats it like Emacs does.
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In POSIX EREs, @samp{^} is always special outside of character alternatives,
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In POSIX EREs, @samp{^} is always special outside of bracket expressions,
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which means the ERE @samp{x^} never matches.
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In Emacs regular expressions, @samp{^} is special only at the
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beginning of the regular expression, or after @samp{\(}, @samp{\(?:}
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@ -3021,7 +3021,7 @@ or @samp{\|}.
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@item
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In POSIX BREs, it is an implementation option whether @samp{$} is special
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before @samp{\)}; GNU @command{grep} treats it like Emacs does.
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In POSIX EREs, @samp{$} is always special outside of character alternatives,
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In POSIX EREs, @samp{$} is always special outside of bracket expressions,
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which means the ERE @samp{$x} never matches.
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In Emacs regular expressions, @samp{$} is special only at the
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end of the regular expression, or before @samp{\)} or @samp{\|}.
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@ -3049,8 +3049,8 @@ character classes @samp{[:ascii:]}, @samp{[:multibyte:]},
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@samp{[:nonascii:]}, @samp{[:unibyte:]}, and @samp{[:word:]}.
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@item
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BRE and ERE alternatives can contain collating symbols and equivalence
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class expressions, e.g., @samp{[[.ch.]d[=a=]]}.
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BREs and EREs can contain collating symbols and equivalence
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class expressions within bracket expressions, e.g., @samp{[[.ch.]d[=a=]]}.
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Emacs regular expressions do not support this.
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@item
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@ -1453,7 +1453,7 @@ and initial semicolons."
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;; are buffer-local, but we avoid changing them so that they can be set
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;; to make `forward-paragraph' and friends do something the user wants.
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;;
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;; `paragraph-start': The `(' in the character alternative and the
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;; `paragraph-start': The `(' in the bracket expression and the
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;; left-singlequote plus `(' sequence after the \\| alternative prevent
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;; sexps and backquoted sexps that follow a docstring from being filled
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;; with the docstring. This setting has the consequence of inhibiting
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@ -383,7 +383,7 @@ Interactively, ARG is the numeric argument, and defaults to 1."
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The syntax for this variable is like the syntax used inside of `[...]'
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in a regular expression--but without the `[' and the `]'.
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It is NOT a regular expression, and should follow the usual
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rules for the contents of a character alternative.
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rules for the contents of a bracket expression.
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It defines a set of \"interesting characters\" to look for when setting
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\(or searching for) tab stops, initially \"!-~\" (all printing characters).
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For example, suppose that you are editing a table which is formatted thus:
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Loading…
Add table
Reference in a new issue