[bug #34772] Backslash escape character ignored for \] inside [ ]

[Eric Blake]

Update of bug #34772 (project grep):

                  Status:                    None => Invalid                
             Open/Closed:                    Open => Closed                 
                 Summary: Backslash escape character ignored for \] inside [ ]
=> Backslash escape character ignored for ] inside [ ]

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Follow-up Comment #1:

I'm closing this as invalid - the only bug here is your mis-understanding of
the documentation.

POSIX requires grep to handle Basic Regular Expressions, as defined at
http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap09.html

In that definition, section 9.3.5 specifies how a bracket expression behaves. 
Among other things, "The <right-square-bracket> ( ']' ) shall lose its special
meaning and represent itself in a bracket expression if it occurs first in the
list (after an initial <circumflex> ( '^' ), if any). Otherwise, it shall
terminate the bracket expression, unless it appears in a collating symbol
(such as "[.].]" ) or is the ending <right-square-bracket> for a collating
symbol, equivalence class, or character class. The special characters '.' ,
'*' , '[' , and '\' ( <period>, <asterisk>, <left-square-bracket>, and
<backslash>, respectively) shall lose their special meaning within a bracket
expression."

Thus, "^[^]]*$' is the regular expression that searches for beginning of line,
followed by any number of characters that match the bracket expression [^]
(basically, any character except backslash), followed by a literal right
bracket, followed by end of line, and grep is correctly interpreting this.

If you want to find all lines that don't contain ], then use the regular
expression "^[^]]*$", which is the beginning of line, followed by the bracket
expression [^]]* (any number of non-right-bracket characters), followed by end
of line.

Your "counterexample" of "^[^[]*$" was not all lines that don't contain [, but
rather all lines that contain neither [ nor backslash.

Other things to think about: why not use grep -v to invert which lines are
printed?  By doing that, you no longer have to use ^, *, or $, since a single
] anywhere in the line excludes that entire line:

ls | grep -v ]

Also, ls | grep is inefficient, especially if you want directory recursion. 
You may want to consider using find for the job, especially since that gives
you recursion capabilities (although then you are dealing with globs rather
than BRE):

find . ! -name '*]*'


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