2a61b3c590
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> |
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