Delete all lines except those containing matches for REGEXP.
A match split across lines preserves all the lines it lies in.
When called from Lisp (and usually interactively as well, see below)
applies to all lines 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.
This command operates on (the accessible part of) all lines whose
accessible part is entirely contained in the region determined by RSTART
and REND. (A newline ending a line counts as part of that line.)
Interactively, in Transient Mark mode when the mark is active, operate
on all lines whose accessible part is entirely contained in the region.
Otherwise, the command applies to all lines starting after point.
When calling this function from Lisp, you can pretend that it was
called interactively by passing a non-nil INTERACTIVE argument.
This function starts looking for the next match from the end of
the previous match. Hence, it ignores matches that overlap a previously found match.