From: Stefan Kangas <stefan@marxist.se>
Date: Sun, 24 Oct 2021 21:56:46 +0000 (+0200)
Subject: * etc/PROBLEMS: Mention problems with regexp matcher.  (Bug#18577)
X-Git-Tag: emacs-28.0.90~185
X-Git-Url: http://git.eshelyaron.com/gitweb/?a=commitdiff_plain;h=0771d8939a;p=emacs.git

* etc/PROBLEMS: Mention problems with regexp matcher.  (Bug#18577)
---

diff --git a/etc/PROBLEMS b/etc/PROBLEMS
index ede83a6e7c0..daff102a0d3 100644
--- a/etc/PROBLEMS
+++ b/etc/PROBLEMS
@@ -742,6 +742,18 @@ completed" message that tls.el relies upon, causing affected Emacs
 functions to hang.  To work around the problem, use older or newer
 versions of gnutls-cli, or use Emacs's built-in gnutls support.
 
+*** Stack overflow in regexp matcher.
+Due to fundamental limitations in the way Emacs' regular expression
+engine is designed, you might run into combinatorial explosions in
+backtracking with certain regexps.
+
+Avoid "\(...\(...\)*...\)*" and "\(...\)*\(...\)*".  Look for a way to
+anchor your regular expression, to avoid matching the null string in
+infinite ways.  The latter is what creates backtrack points, and
+eventual overflow in practice.
+
+(Also prefer "\(?:...\)" to "\(...\)" unless you need the latter.)
+
 * Runtime problems related to font handling
 
 ** Characters are displayed as empty boxes or with wrong font under X.