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Document regular expression special cases better

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In particular, document that escape sequences like \b*
are currently buggy.
This commit is contained in:
Paul Eggert 2023-06-19 11:09:00 -07:00
parent c5f819aa03
commit d84b026dbe
1 changed files with 18 additions and 10 deletions
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28
doc/lispref/searching.texi
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@ -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{]}.