Re: what is the regular expression that matches nothing? -- I think it is the null expression. When I do "egrep '' somefile" the result is the same as when I do "cat somefile". The result of "egrep -v '' somefile" is, as expected, nothing. HTH

[Stan Brown]

There is no regular expression that matches nothing.

I you want to match a zero-length line, use
        ^$

But the idea of matching nothing has an inherent contradiction. Why 
don't you tell us what you're actually trying to accomplish?

-- 
Stan Brown, Oak Road Systems, Tompkins County, New York, USA
                                  http://OakRoadSystems.com/
"Don't move, or I'll fill you full of [... pause ...] little
yellow bolts of light."           -- Farscape, first episode