Stowage/go
2
0
Fork 0
mirror of https://github.com/golang/go.git synced 2025-12-08 06:10:04 +00:00
Code Issues Projects Releases Packages Wiki Activity

regexp: fix repeat of preferred empty match

Browse source 
In Perl mode, (|a)* should match an empty string at the start of the
input. Instead it matches as many a's as possible.
Because (|a)+ is handled correctly, matching only an empty string,
this leads to the paradox that e* can match more text than e+
(for e = (|a)) and that e+ is sometimes different from ee*.

This is a very old bug that ultimately derives from the picture I drew
for e* in https://swtch.com/~rsc/regexp/regexp1.html. The picture is
correct for longest-match (POSIX) regexps but subtly wrong for
preferred-match (Perl) regexps in the case where e has a preferred
empty match. Pointed out by Andrew Gallant in private mail.

The current code treats e* and e+ as the same structure, with
different entry points. In the case of e* the preference list ends up
not quite in the right order, in part because the “before e” and
“after e” states are the same state. Splitting them apart fixes the
preference list, and that can be done by compiling e* as if it were
(e+)?.

Like with any bug fix, there is a very low chance of breaking a
program that accidentally depends on the buggy behavior.

RE2, Go, and Rust all have this bug, and we've all agreed to fix it,
to keep the implementations in sync.

Fixes #46123.

Change-Id: I70e742e71e0a23b626593b16ddef3c1e73b413b0
Reviewed-on: https://go-review.googlesource.com/c/go/+/318750
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
This commit is contained in:
Russ Cox 2021-05-11 10:52:44 -04:00
parent fd4631e24f
commit 2a61b3c590
8 changed files with 176 additions and 46 deletions
Download patch file Download diff file Expand all files Collapse all files

18
src/regexp/testdata/nullsubexpr.dat vendored
View file

@ -1,8 +1,7 @@
NOTE null subexpression matches : 2002-06-06
E (a*)* a (0,1)(0,1)
#E SAME x (0,0)(0,0)
E SAME x (0,0)(?,?) RE2/Go
E SAME x (0,0)(0,0)
E SAME aaaaaa (0,6)(0,6)
E SAME aaaaaax (0,6)(0,6)
E (a*)+ a (0,1)(0,1)
@ -19,8 +18,7 @@ E SAME aaaaaa (0,6)(0,6)
E SAME aaaaaax (0,6)(0,6)
E ([a]*)* a (0,1)(0,1)
#E SAME x (0,0)(0,0)
E SAME x (0,0)(?,?) RE2/Go
E SAME x (0,0)(0,0)
E SAME aaaaaa (0,6)(0,6)
E SAME aaaaaax (0,6)(0,6)
E ([a]*)+ a (0,1)(0,1)
@ -28,8 +26,7 @@ E SAME x (0,0)(0,0)
E SAME aaaaaa (0,6)(0,6)
E SAME aaaaaax (0,6)(0,6)
E ([^b]*)* a (0,1)(0,1)
#E SAME b (0,0)(0,0)
E SAME b (0,0)(?,?) RE2/Go
E SAME b (0,0)(0,0)
E SAME aaaaaa (0,6)(0,6)
E SAME aaaaaab (0,6)(0,6)
E ([ab]*)* a (0,1)(0,1)
@ -41,11 +38,9 @@ E SAME bbbbbb (0,6)(0,6)
E SAME aaaabcde (0,5)(0,5)
E ([^a]*)* b (0,1)(0,1)
E SAME bbbbbb (0,6)(0,6)
#E SAME aaaaaa (0,0)(0,0)
E SAME aaaaaa (0,0)(?,?) RE2/Go
E SAME aaaaaa (0,0)(0,0)
E ([^ab]*)* ccccxx (0,6)(0,6)
#E SAME ababab (0,0)(0,0)
E SAME ababab (0,0)(?,?) RE2/Go
E SAME ababab (0,0)(0,0)
E ((z)+|a)* zabcde (0,2)(1,2)
@ -65,8 +60,7 @@ B \(a*\)*\(x\)\(\1\) axa (0,3)(0,1)(1,2)(2,3)
B \(a*\)*\(x\)\(\1\)\(x\) axax (0,4)(0,1)(1,2)(2,3)(3,4)
B \(a*\)*\(x\)\(\1\)\(x\) axxa (0,3)(1,1)(1,2)(2,2)(2,3)
#E (a*)*(x) x (0,1)(0,0)(0,1)
E (a*)*(x) x (0,1)(?,?)(0,1) RE2/Go
E (a*)*(x) x (0,1)(0,0)(0,1)
E (a*)*(x) ax (0,2)(0,1)(1,2)
E (a*)*(x) axa (0,2)(0,1)(1,2)