Re: [musl] Re: Different results with regex.h between Musl and Libc

[Rich Felker <dalias@libc.org>]

On Mon, Jun 10, 2024 at 02:47:04PM +0300, Valery Ushakov wrote:
> On Mon, Jun 10, 2024 at 05:38:36 +0000, Nigel Kukard wrote:
> 
> > Musl output (Alpine 3.20), musl-1.2.5-r1...
> > 
> > The input '37' matches the pattern '^([0-9]*)?\.?([0-9]*)?$'
> > Match 0: 37
> > Match 1:
> > Match 2: 37
> > 
> > Glibc output (ArchLinux), glibc 2.39+r52+gf8e4623421-1...
> > 
> > The input '37' matches the pattern '^([0-9]*)?\.?([0-9]*)?$'
> > Match 0: 37
> > Match 1: 37
> > Match 2:
> 
> I'm not sure what POSIX requires here.  The closest I can find after
> skimming through "9. Regular Expressions" is 9.4.6 that ends with:
> 
>   An ERE matching a single character repeated by an '*', '?', or an
>   interval expression shall not match a null expression unless this is
>   the only match for the repetition or it is necessary to satisfy the
>   exact or minimum number of occurrences for the interval expression.
> 
>     https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap09.html#tag_09_04_06
> 
> I'm not sure what to read into the absense of the usual "or an ERE
> enclosed in parentheses" chorus here.

This looks like a bug. The general requirement (from memory; I don't
have the spec in front of me now) is that each subexpression, in order
from the beginning of the regex, matches the maximal-length input it
can, subject to the overall constraint that the entire regex match the
earliest (first priority) and maximal length (second priority) input
it can.

I guess we need to dig into why this is happening, ensure it's
actually incorrect, and figure out how to fix it...

Rich