[SOLVED] Exclamation point in Grep for Negated Lookbehind: How to Do it?

mattseanbachman · 03-15-2010, 02:13 PM

I'm trying to work with lookaround using grep with the -P switch. Here's where the problem lies:

Code:

grep -P "wall(?=street)" ./words 
wallstreet

grep -P "wall(?!street)" ./words 
bash: !street: event not found

In the first case, everything works great. But when I try to do negative lookahead, i.e. if street is right after wall, discard the result, it doesn't work.

I've tried escaping the exclamation point. Here's what that does:

Code:

grep -P "wall(?\!street)" ./words 
grep: unrecognized character after (? or (?-

I've also Googled this and checked forums; this has probably been covered somewhere else but I am at a loss to find where.

Anyone have any comments on why it's doing this or how to fix it?

GrapefruiTgirl · 03-15-2010, 02:23 PM

Seems to work for me by using single quotes around the thing you're grepping:

Code:

root@reactor: echo 'wallstreet' | grep -P 'wall(?=street)'
wallstreet

root@reactor: echo 'wallstreet' | grep -P 'wall(?!street)'
root@reactor:

Does that seem right? Grep acts differently when things are double-quoted (I'm sure there's a fully-detailed explanation, with lots of examples, but rather than diagnose it, I usually just switch to single quotes and try that.

)

Sasha

pixellany · 03-15-2010, 02:24 PM

From the GREP man page:

Quote:

-P, --perl-regexp
Interpret PATTERN as a Perl regular expression.
This is highly experimental and grep -P may warn of
unimplemented features.

For me at least, that says: "try a different approach." In this case maybe just use Perl?

GrapefruiTgirl · 03-15-2010, 02:27 PM

Yes, or use two instances of grep, connected with a pipe (klunky maybe, but whatever you like):

Code:

echo 'wallstreet' | grep wall | grep -v wallstreet

Sasha

mario.almeida · 03-15-2010, 02:29 PM

Hi,

also this way should work.

x='!street'; grep -P "wall(?${x})" ./words

mattseanbachman · 03-15-2010, 07:42 PM

Thanks for everybody's help. All solutions (from GrapefruiTgirl and mario.almeida) worked.

This might very well be a case of the -P switch being something to be careful with...however, this is the first bug that I've encountered using it thusfar. Mostly when I posted this I thought it was a syntax problem or what-not. Glad to know I wasn't out of my mind!

Again, thanks for the help everyone.

syg00 · 03-15-2010, 08:10 PM

Quote:

Originally Posted by GrapefruiTgirl

Does that seem right? Grep acts differently when things are double-quoted

That's interpolation for you ...
Its everywhere - try this

Code:

echo  'fred\\\ngeorge' ; echo "fred\\\ngeorge"

GrapefruiTgirl · 03-15-2010, 08:45 PM

@ syg00 -- yep, I understand, but I don't believe interpolation is the right word for it (not to my understanding if the word) but that's neither here nor there

--

Quote:

in·ter·po·late /ɪnˈtɜrpəˌleɪt/ [in-tur-puh-leyt]
–verb (used with object) -- to cause oneself (object) to become the subject of an Interpol investigation. To interpolate oneself..

I thought it was more simple- maybe 'shell interpretation' or something like this..

@ mattseanbachman, it's not a bug; more of a behavior. The different types of quotes cause the shell to interpret, expand, ignore, etc., the stuff within the quotes.

Anyhow, on with the show!