Function: flush-lines

Delete lines containing matches for REGEXP.
When called from Lisp (and usually when called interactively as
well, see below), applies to the part of the buffer after point.
The line point is in is deleted if and only if it contains a
match for regexp starting after point.

If REGEXP contains upper case characters (excluding those preceded by `\')
and `search-upper-case' is non-nil, the matching is case-sensitive.

Second and third arg RSTART and REND specify the region to operate on.
Lines partially contained in this region are deleted if and only if
they contain a match entirely contained in it.

Interactively, in Transient Mark mode when the mark is active, operate
on the contents of the region. Otherwise, operate from point to the
end of (the accessible portion of) the buffer. When calling this function
from Lisp, you can pretend that it was called interactively by passing
a non-nil INTERACTIVE argument.

If a match is split across lines, all the lines it lies in are deleted.
They are deleted _before_ looking for the next match. Hence, a match starting on the same line at which another match ended is ignored.