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Date: Mon, 02 Sep 2013 17:54:11 +0300
From: Andriy Gapon <avg@FreeBSD.org>
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To: FreeBSD Current <freebsd-current@FreeBSD.org>,
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Subject: bug with special bracket expressions in regular expressions
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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?
Thank you!
-- 
Andriy Gapon