bug with special bracket expressions in regular expressions

Damian Weber dweber at htw-saarland.de
Mon Sep 2 15:41:04 UTC 2013

On Mon, 2 Sep 2013, Andriy Gapon wrote:

> re_format(7) says:
>      There are two special cases? of bracket expressions: the bracket expres?
>      sions ?[[:<:]]? and ?[[:>:]]? match the null string at the beginning and
>      end of a word respectively.  A word is defined as a sequence of word
>      characters which is neither preceded nor followed by word characters.  A
>      word character is an alnum character (as defined by ctype(3)) or an
>      underscore.  This is an extension, compatible with but not specified by
>      IEEE Std 1003.2 (?POSIX.2?), and should be used with caution in software
>      intended to be portable to other systems.
> However I observe the following:
> $ echo "cd0 cd1 xx" | sed 's/cd[0-9][^ ]* *//g'
> xx
> $ echo "cd0 cd1 xx" | sed 's/[[:<:]]cd[0-9][^ ]* *//g'
> cd1 xx
> In my opinion '[[:<:]]' should not affect how the pattern is matched in this case.
> Any thoughts, suggestions?

there are two simpler expressions, whose difference I don't understand either
(tested on 8.4-PRERELEASE)

$ echo "cd0 cd1 xx" | sed 's/cd[0-9] //g'
$ echo "cd0 cd1 xx" | sed 's/[[:<:]]cd[0-9] //g'
cd1 xx

-- Damian

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