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From: Nick Owens <mischief@offblast.org>
Date: Fri, 11 Jan 2019 05:10:19 -0800
Message-ID: <CAH_zEu4CUeicUE+TN5NgtqHZjtF6YQAzb1JcyS+ZLJv6T2fMxg@mail.gmail.com>
Subject: Re: [9front] bug in sed
To: 9front@9front.org
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X-Bullshit: encrypted core-aware API app high-performance interface 

in sed.c:/^substitute, when g is set, sed begins doing
match/substitute in a loop. after a substitution it moves the pointer
into the current string (called 'loc2') forward and tries to match
again. however, when it attempts to match the regexp again, the regexp
library is unaware that the string to be matched is not the true
beginning of the line. thus the match for ^ always succeeds.

compare OpenBSD regexec and sed, which use a REG_NOTBOL flag to
indicate that ^ cannot match in subsequent matches

https://man.openbsd.org/man3/regex.3
https://github.com/openbsd/src/blob/master/usr.bin/sed/process.c#L334

On Fri, Jan 11, 2019 at 4:58 AM hiro <23hiro@gmail.com> wrote:
>
> here's another testcase without needing spaces
>
> cpu% cat > test
> asdfasdf
> xasdf
>  cpu% sed  's/^asdf//g' test
>
> xasdf
> cpu% ssam  's/^asdf//g' test
> asdf
> xasdf