Re: grep bug with -P -o option

[Davide Brini]

On Thursday 15 Jul 2010 03:43:31 fd wrote:
> Dear all
> 
> I'm a novice in this Linux business, but I think there's something not
> right. When I try to search using a perl regular expression, so if a
> do:
> 
> gcc --help | grep -P  '(?<=the )'
> 
> I get all lines for which there is any text preceded by "the "
> 
> 
>   -dumpspecs               Display all of the built in spec strings
>   -dumpversion             Display the version of the compiler
>   -dumpmachine             Display the compiler's target processor
>   -print-search-dirs       Display the directories in the compiler's
> search path
>   -print-libgcc-file-name  Display the name of the compiler's
> companion library
>   -print-file-name=<lib>   Display the full path to library <lib>
>   -print-prog-name=<prog>  Display the full path to compiler component
> <prog>
>   -print-multi-directory   Display the root directory for versions of
> libgcc
>   -print-multi-lib         Display the mapping between command line
> options and
> ...
> 
> If I introduce the -o option so that I have only the part of each line
> that matches a pattern I have no result
> 
> gcc --help | grep -P -o '(?<=the )'
> 
> Is this a bug, or am I doing something wrong.

The lookbehind is a zero-width assertion. Think of it as a boolean condition.

If you want the text that follows the (first) "the ", then do

grep -P -o '(?<=the ).*'

-- 
D.