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grep branch, master, updated. v3.3-55-geac1e4d

From: Paul Eggert
Subject: grep branch, master, updated. v3.3-55-geac1e4d
Date: Mon, 30 Dec 2019 03:48:54 -0500 (EST)

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      from  a517df5bd51e68c1290ec596b87e8fbe7a381279 (commit)

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commit eac1e4d50a73bb33c35e5f9f95a201e22d295827
Author: Paul Eggert <address@hidden>
Date:   Mon Dec 30 00:48:20 2019 -0800

    doc: mention back-reference bugs
    Inspired by Bug#26864.
    * doc/grep.texi (Known Bugs): New section.
    Mention back-reference issues.

diff --git a/doc/grep.texi b/doc/grep.texi
index 42832ab..8866ec4 100644
--- a/doc/grep.texi
+++ b/doc/grep.texi
@@ -1536,6 +1536,8 @@ When multiple regular expressions are given with
 @option{-e} or from a file (@samp{-f @var{file}}),
 back-references are local to each expression.
+@xref{Known Bugs}, for some known problems with back-references.
 @node Basic vs Extended
 @section Basic vs Extended Regular Expressions
 @cindex basic regular expressions
@@ -1965,6 +1967,11 @@ GNU bug report logs for @command{grep}}.
 If you find a bug not listed there, please email it to
 @email{} to create a new bug report.
+* Known Bugs::
+@end menu
+@node Known Bugs
 @section Known Bugs
 @cindex Bugs, known
@@ -1974,7 +1981,17 @@ In addition, certain other
 obscure regular expressions require exponential time and
 space, and may cause @command{grep} to run out of memory.
-Back-references are very slow, and may require exponential time.
+Back-references can greatly slow down matching, as they can generate
+exponentially many matching possibilities that can consume both time
+and memory to explore.  Also, the POSIX specification for
+back-references is at times unclear.  Furthermore, many regular
+expression implementations have back-reference bugs that can cause
+programs to return incorrect answers or even crash, and fixing these
+bugs has often been low-priority---for example, as of 2019 the GNU C
+library bug database contained back-reference bugs 52, 10844, 11053,
+and 23522, with little sign of forthcoming fixes.  Luckily,
+back-references are rarely useful and it should be little trouble to
+avoid them in practical applications.
 @node Copying


Summary of changes:
 doc/grep.texi | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)


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