[TriLUG] regex: match lines NOT containing X

[Len Boyle]

$ cat tempe
If two heads are better than one,
are two keyboards better than one?
Would you say the same if it were two hydra heads?
$ grep -v heads tempe
are two keyboards better than one?
$

-----Original Message-----
From: trilug-bounces at trilug.org [mailto:trilug-bounces at trilug.org] On Behalf Of Kevin Hunter
Sent: Wednesday, February 22, 2012 9:56 AM
To: Triangle Linux Users Group General Discussion
Subject: [TriLUG] regex: match lines NOT containing X

Hullo List,

An actual technical question here: Is there a way to match an entire line such that the entire line does *not* contain a sequence of characters?

Example input containing three lines:
-----
If two heads are better than one,
are two keyboards better than one?
Would you say the same if it were two hydra heads?
-----

My criteria is very specific: I want lines that do *not* contain 'heads'.  My first thought was to use an assertion:

/^.*two.*(?!heads).*$/

However, this naive implementation matches both lines 2 and 3.  I've needed this construct more than a few times, so I'm almost sure it exists; I just don't know the right Google terms, apparently.  On the other hand, I could accept that this may not be possible in a single step regex.

Any ideas?

Thanks,

Kevin
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