POSIX regex VS. multi-byte characters

Gabor Kovesdan gabor at FreeBSD.org
Fri Sep 2 02:26:10 UTC 2011

Hi Folks,

While working on bringing in a new regex code to FreeBSD, I came into an 
issue. POSIX says here: 

"Matching shall be based on the bit pattern used for encoding the 
character, not on the graphic representation of the character. This 
means that if a character set contains two or more encodings for a 
graphic symbol, or if the strings searched contain text encoded in more 
than one codeset, no attempt is made to search for any other 
representation of the encoded symbol. If that is required, the user can 
specify equivalence classes containing all variations of the desired 
graphic symbol."

According to my interpretation of this text, if someone specifies a 
single bit as pattern that can be a prefix of a multi-byte character 
that shall match, since match is based on bit pattern not semantical 
meaning. Besides, in a consistent environment that uses a single 
encoding and also supposing a user with common sense that would not 
enter meaningless input, only whole characters should occur in the 
pattern. However, GNU grep has a test in its regression test suite that 
contradicts to this and chooses the opposite approach, i.e. it shall not 
match a fragment of a character. Looking at the standard, I think GNU 
grep is incorrect and my interpretation is the correct one.

Could you please comment on this?

Gabor Kovesdan

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