grep -P

tomazos has asked for the wisdom of the Perl Monks concerning the following question:

In the man page for grep it says:

 -P, --perl-regexp
              Interpret PATTERN as a Perl regular expression.
[download]

When I try this I get:

$ grep -P 'whatever'
grep: The -P option is not supported
[download]

(Using Ubuntu 6.06 Dapper Drake)

Any ideas on the history of this feature? Why is it "not supported"? How do I get a version of grep that supports it?

When the feature works, what is the extent of greps support for Perl regexps? Does it actually use perl, or is it a reimplemented engine?

-Andrew.

Comment on grep -P Select or Download Code

Replies are listed 'Best First'.
Re: grep -P by ambrus (Abbot) on Jun 25, 2006 at 19:58 UTC
It uses an external library called pcre, and I think the feature is not always compiled in grep. If you want to use real perl expressions with grep, an alternative to gnu grep is to use one of the several grep clones implemented in perl, such as mine.	[reply]
Re^2: grep -P by tomazos (Deacon) on Jun 25, 2006 at 21:25 UTC
Interesting... `$ apt-cache search pcre libpcre3 - Perl 5 Compatible Regular Expression Library - runtime file +s libpcre3-dev - Perl 5 Compatible Regular Expression Library - developm +ent files libpcrecpp0 - Perl 5 Compatible Regular Expression Library - C++ runti +me files pcregrep - grep utility that uses perl 5 compatible regexes. pgrep - Dummy package for transition to pcregrep` [download] I suppose pcregrep or pgrep might somehow help. -Andrew.	[reply] [d/l]
Re: grep -P by Fletch (Bishop) on Jun 25, 2006 at 22:11 UTC
Keep in mind that even if it's compiled against `pcre` that it's not going to be able to understand all Perl regexen syntax (things like `(?{})`, for instance). I always keep a copy of `tcgrep` lying around.	[reply] [d/l] [select]
Re: grep -P by sgifford (Prior) on Jun 26, 2006 at 01:46 UTC
If you don't need the more complex command-line options of `grep`, you can just use `perl -ne 'print if (/pat/)'`. Even the trickier options usually come down to just a small variation on that theme. -- sgifford's Web page	[reply] [d/l]
Re^2: grep -P by tomazos (Deacon) on Jun 26, 2006 at 03:08 UTC
I guess -R (recurse directory) and -l (print filenames containing matches) are two that I use often. The one liner is a bit of a mouthful. But just for fun: `perl -MFile::Find -ne 'find(& sub { $f = 1; if (/pat/ and $f) { $f = 0 +; print $File::Find::name . "\n" }, "mydir");'` [download] ...or something like that. -Andrew.	[reply] [d/l]


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