[TriLUG] regex: match lines NOT containing X

[Kevin Hunter]

At 10:26am -0500 Wed, 22 Feb 2012, Alexey Toptygin wrote:
> If you can't invert the whole regex (as in grep -v or !~ in perl),
> you can do:
>
> /^(?:[^h]|h[^e]|he[^a]|hea[^d]|head[^s])*$/

This finally set me off on the right track.  I can't invert the whole 
regex, but I do know that no where in the string can I match against 
heads.  Thanks to the above, it dawned on me that zero-width assertion 
must be true for /every/ character matched.  So (recopying context from 
earlier message), to match only the lines that don't have heads in them 
with a single regex (and exploiting some knowledge of the structure that 
'two' will always come before 'heads', if 'heads' comes at all):

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

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

And voila!  It's the grouping of (.(?!something)) that's the key.

Thanks all!

Kevin