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| author | Paul Eggert <eggert@cs.ucla.edu> | 2019-12-30 00:22:05 -0800 |
|---|---|---|
| committer | Paul Eggert <eggert@cs.ucla.edu> | 2019-12-30 00:22:32 -0800 |
| commit | 35cddb5464e1aec6fdd3c89a7e43d4374aab31e6 (patch) | |
| tree | a00a11180bc79fc40a049746a9c9f2dfc7b49938 /doc/regex.texi | |
| parent | ff6ee46a283ef023146b4e69fc9cf776ac546899 (diff) | |
| download | gnulib-35cddb5464e1aec6fdd3c89a7e43d4374aab31e6.tar.gz | |
doc: document trouble with back-references
* doc/regex.texi (Back-reference Operator): Mention bugs etc.
Diffstat (limited to 'doc/regex.texi')
| -rw-r--r-- | doc/regex.texi | 12 |
1 files changed, 12 insertions, 0 deletions
diff --git a/doc/regex.texi b/doc/regex.texi index 7b83cdd8e1..4e0da9b397 100644 --- a/doc/regex.texi +++ b/doc/regex.texi @@ -1144,6 +1144,18 @@ example, @samp{(a(b))\2*} matches @samp{a} followed by two or more 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 |