Re: sed bug

tags 20138 notabug
thanks

On Wed, Mar 18, 2015 at 1:39 PM, Bob Proulx <[email protected]> wrote:
> Jim Meyering wrote:
>> Peter Scott wrote:
>> > # This works
>> > #
>> > $ sed 's/foo/bar/i'
>> > FOO
>> > bar
>> > $
>> >
>> > # I think this should work too.
>> > #
>> > $ sed '/foo/s//bar/i'
>> > sed: -e expression #1, char 13: cannot specify modifiers on empty regexp
>>
>> Thanks for spotting and reporting that. I certainly think it should work.
>> I suspect it is simply an oversight, and we will fix it for the
>> upcoming release.
>
> Why should that work?  The previous regular expression is repeated.
> Therefore any case insensitive flags must be attached to the original
> expression not the repeat of the original expression.
>
> Isn't that a documented behavior?
>
>      The empty regular expression `//' repeats the last regular
>      expression match (the same holds if the empty regular expression is
>      passed to the `s' command).  Note that modifiers to regular
>      expressions are evaluated when the regular expression is compiled,
>      thus it is invalid to specify them together with the empty regular
>      expression.
>
> Plus 'i' isn't a traditional sed flag.  It isn't in the legacy Unix
> sed.  GNU sed uses 'I' for that purpose, avoiding 'i' already used for
> insert.
>
>   `/REGEXP/I'
>   `\%REGEXP%I'
>      The `I' modifier to regular-expression matching is a GNU extension
>      which causes the REGEXP to be matched in a case-insensitive manner.
>
> Therefore the failure makes sense and is documented as such.  The way
> to do this action is this following way.  Place the case-insensitive
> flag for the regular expression to be matched in the pattern and then
> it is applied when it is repeated using the empty expression
> shorthand.
>
>   $ echo FOO | sed '/foo/Is//bar/'
>   bar
>
> This allows separate control of the address range pattern and the
> substitution pattern.
>
>   $ echo FOO | sed '/foo/Is/f/b/I'
>   bOO
>
> Unfortunately the 'i' command is already used for inserting lines.
> Therefore using 'i' often surprises people by yielding an insertion,
> as it should do in retrospect, as a command instead of being a flag.
>
>   $ echo foo | sed '/foo/is/baz/bar/'
>   s/baz/bar/
>
> The /foo/ matches and therefore the insert action is triggered.
>
>   `i\'
>   `TEXT'
>      As a GNU extension, this command accepts two addresses.
>
>      Immediately output the lines of text which follow this command
>      (each but the last ending with a `\', which are removed from the
>      output).
>
> Since I think that is somewhat confusingly written here is the old sed
> doc for that part.  (Where (1) means 1 address max there.)
>
>      (1)i\
>      <text> -- insert lines
>           The i function  behaves identically to the a function,
>           except  that <text> is written to the output before the
>           matched line.  All other comments about the a function
>           apply to the i function as well.

Well, that will teach me to reply before looking at code/documentation.
Thanks, Bob! It is clearly not a bug after all.