Regex character and collation class documentation

James B. Byrne byrnejb at
Fri Nov 10 17:37:10 UTC 2017

On Thu, November 9, 2017 16:36, mfv wrote:
>> On Wed, 2017-11-08 at 12:47 "James B. Byrne via freebsd-questions"
>>However I see no reference to [.NUL.] anywhere.  The sed man page has
>>no reference to nul or NUL at all and tr only has this to say:
>>     The tr utility has historically not permitted the manipulation
>>     of NUL bytes in its input and, additionally, stripped NUL's from
>>     its input stream.  This implementation has removed this behavior
>>     as a bug.
>>Is there a master list of character/collation classes for FreeBSD
>>regex?  I have read the man pages for grep and re_format.  In no case
>>is the character or collation class NUL mentioned.
>>Where is the usage of [.NUL.] documented?
> Hello James,
> This may help you with a bit of hacking.
> I asked myself the same question but could not find a satisfactory
> answer.  After remembering that "man ascii" has names for all
> non-printable ASCII characters, I placed some of these characters in a
> text file and then removed the same characters using their name.
> Thus:
>  - the character ^@ was removed using [[.NUL.]]
>  - the character ^G was removed using [[.BEL.]]
>  - the character ^F was removed using [[.ACK.]]
>  - etc,
> I did not try all non-printable characters but a large sampling
> followed this pattern.  Trying to use SP for a space produced the
> following error:
> sed: 1: "/[[.SP.]]/d": RE error: invalid collating element
> Perhaps there are other exceptions similar to SP.
> This syntax also recognises printable characters as well.  For example
> the character 'A' was removed using 's/[[.A.]]//g'.
> I would have preferred some formal documentation on this matter but
> like yourself am still searching.
> Cheers ...
> Marek

Thank you.  I discovered that a [.<symbol>.] collation reference
pertains to the active LOCALE setting as defined by LC_ALL. At least
so I find in the documentation I have read.  But I would not have
thought to look in man ascii for the answer to my question.

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