Re: Unexpected regex behaviour

[Rich Felker <dalias@libc.org>]

On Mon, Oct 29, 2018 at 11:26:19PM +0100, Robert Högberg wrote:
> Hi,
> 
> I've noticed that the musl regex implementation behaves slightly
> differently than the glibc implementation. I'm attaching a short program
> showing the behaviour.
> 
> The difference makes yate (http://yate.null.ro) misbehave when running with
> musl (reported here: https://github.com/openwrt/telephony/issues/378).
> 
> Yate uses a regexp like this:
> "^\\([[:alpha:]][[:alnum:]]\\+:\\)\\?/\\?/\\?\\([^[:space:][:cntrl:]@]\\+@\\)\\?\\([[:alnum:]._+-]\\+\\|[[][[:xdigit:].:]\\+[]]\\)\\(:[0-9]\\+\\)\\?"
> 
> ... to parse strings like:
> "sip:012345678@11.111.11.111:5060;user=phone"
> 
> ... and the matches produced by musl are:
> Match 0:  0 - 32        sip:012345678@11.111.11.111:5060
> Match 1: -1 - -1
> Match 2:  0 - 14        sip:012345678@
> Match 3: 14 - 27        11.111.11.111
> Match 4: 27 - 32        :5060
> 
> ... while glibc produces:
> Match 0:  0 - 32        sip:012345678@11.111.11.111:5060
> Match 1:  0 -  4        sip:
> Match 2:  4 - 14        012345678@
> Match 3: 14 - 27        11.111.11.111
> Match 4: 27 - 32        :5060
> 
> What do you think?
> 
> I've only tested musl 1.1.19. Sorry if this is not valid for later
> releases. I skimmed the 1.1.20 release notes and didn't find anything regex
> related.

I haven't checked which of the extensions you're using are supported
in musl, but the above is not a conforming POSIX BRE. It would be a
lot more readable and portable to use POSIX ERE (REG_EXTENDED) which
has the +, ?, and | operators as standard features. This looks like it
should work:

"^([[:alpha:]][[:alnum:]]+:)?/?/?([^[:space:][:cntrl:]@]+@)?([[:alnum:]._+-]+|[[][[:xdigit:].:]+[]])(:[0-9]+)?"

The only reason to use POSIX BRE is if you need backreferences, which
are not regular and explicitly not supported in ERE.

Rich