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master d84b026dbef 1/3: Document regular expression special cases better
From: |
Paul Eggert |
Subject: |
master d84b026dbef 1/3: Document regular expression special cases better |
Date: |
Mon, 19 Jun 2023 14:09:14 -0400 (EDT) |
branch: master
commit d84b026dbefce6604a35a83131649291a74fda67
Author: Paul Eggert <eggert@cs.ucla.edu>
Commit: Paul Eggert <eggert@cs.ucla.edu>
Document regular expression special cases better
In particular, document that escape sequences like \b*
are currently buggy.
---
doc/lispref/searching.texi | 28 ++++++++++++++++++----------
1 file changed, 18 insertions(+), 10 deletions(-)
diff --git a/doc/lispref/searching.texi b/doc/lispref/searching.texi
index b8d9094b28d..3970faebbf3 100644
--- a/doc/lispref/searching.texi
+++ b/doc/lispref/searching.texi
@@ -505,9 +505,10 @@ beginning of a line.
When matching a string instead of a buffer, @samp{^} matches at the
beginning of the string or after a newline character.
-For historical compatibility reasons, @samp{^} can be used only at the
-beginning of the regular expression, or after @samp{\(}, @samp{\(?:}
-or @samp{\|}.
+For historical compatibility, @samp{^} is special only at the beginning
+of the regular expression, or after @samp{\(}, @samp{\(?:} or @samp{\|}.
+Although @samp{^} is an ordinary character in other contexts,
+it is good practice to use @samp{\^} even then.
@item @samp{$}
@cindex @samp{$} in regexp
@@ -519,8 +520,10 @@ matches a string of one @samp{x} or more at the end of a
line.
When matching a string instead of a buffer, @samp{$} matches at the end
of the string or before a newline character.
-For historical compatibility reasons, @samp{$} can be used only at the
+For historical compatibility, @samp{$} is special only at the
end of the regular expression, or before @samp{\)} or @samp{\|}.
+Although @samp{$} is an ordinary character in other contexts,
+it is good practice to use @samp{\$} even then.
@item @samp{\}
@cindex @samp{\} in regexp
@@ -540,12 +543,17 @@ example, the regular expression that matches the @samp{\}
character is
@samp{\} is @code{"\\\\"}.
@end table
-@strong{Please note:} For historical compatibility, special characters
-are treated as ordinary ones if they are in contexts where their special
-meanings make no sense. For example, @samp{*foo} treats @samp{*} as
-ordinary since there is no preceding expression on which the @samp{*}
-can act. It is poor practice to depend on this behavior; quote the
-special character anyway, regardless of where it appears.
+For historical compatibility, a repetition operator is treated as ordinary
+if it appears at the start of a regular expression
+or after @samp{^}, @samp{\(}, @samp{\(?:} or @samp{\|}.
+For example, @samp{*foo} is treated as @samp{\*foo}, and
+@samp{two\|^\@{2\@}} is treated as @samp{two\|^@{2@}}.
+It is poor practice to depend on this behavior; use proper backslash
+escaping anyway, regardless of where the repetition operator appears.
+Also, a repetition operator should not immediately follow a backslash escape
+that matches only empty strings, as Emacs has bugs in this area.
+For example, it is unwise to use @samp{\b*}, which can be omitted
+without changing the documented meaning of the regular expression.
As a @samp{\} is not special inside a character alternative, it can
never remove the special meaning of @samp{-}, @samp{^} or @samp{]}.