+2019-12-30 Paul Eggert <eggert@cs.ucla.edu>
+
+ doc: document trouble with back-references
+ * doc/regex.texi (Back-reference Operator): Mention bugs etc.
+
2019-12-29 Paul Eggert <eggert@cs.ucla.edu>
doc: use “back-reference” for \1 etc.
If there is no preceding @w{@var{digit}-th} subexpression, the regular
expression is invalid.
+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 Anchoring Operators
@section Anchoring Operators