Regex / Vim: Matching everything except a pattern, where pattern is multi-line? - regex

Question

Is there a way in Vim to perform a similar operation to
:v/PATTERN/d
where PATTERN is a multi-line regex pattern? I'm sure there is a way to do this in script, but I am mainly curious as to if it is possible to do using only standard regex substitution or Vim commands, because at this point it is more academic than an actual need.
My example is the following:
asdf
begin
blah
end
asdf
alsdfjasf
begin
random stuff
end
...
I want to get the blocks of begin/end with the lines between them, but ignore everything outside of the blocks, ultimately ending up with
begin
blah
end
begin
random stuff
end
...
My thoughts were to do
:v/begin\_.\{-}end/d
where everything didn't match that would be deleted or even copied to register, but obviously :v and :g only work on single lines.
Then, I started going down the path of running a substitute and substitute everything with empty string that DIDN'T match the begin\_.\{-}end pattern, but I cannot grasp how to achieve such using look-behinds or anything. The regex works perfectly fine when just searching, but I can't figure out how to tell the regex engine to find everything BUT that pattern. Any ideas?

Answer 1

clear reg a
qaq
append begin...end to reg a
:g/begin/,/end/y A
open new tab
:tabnew
put reg a
"ap

Answer 2

Flip it inside out, and delete everything delimited by the pattern:
:%s/\%(^end\n*\|\%^\)\zs\_.\{-}\ze\%(^begin\|\%$\)//
\%( ... ) - non capturing group
^end\n* - the end of your pattern
\%^ - the beginning of the file
\zs - don't include anything matched before this point in the string to be replaced
\_.\{-} - non-greedy matching of anything (including newlines)
\ze - don't include anything matched after this point in the string to be replaced
^begin - the beginning of your pattern
\%$ - the end of your pattern

Answer 3

The commands executed by g and v can themselves take ranges, so you can act on everything from "begin" to "end" with :g /begin/ .,/end/xxx where xxx is a command to execute on that range. You cant really use this for :v but there are several ways you could do it in multiple passes, e.g.
"mark all the lines we want to keep by putting '#' at the start of the line"
:g/^begin/ .,/^end/ s/^/#/
"delete all unmarked lines"
:v/^#/ d
"remove the markers"
:s/^#//
Of course this assumes you do not have any lines in the file starting with #.
Alternatively you could delete everything between each "end" and the next "begin"
:g/^end/ +1,/^begin/-1 d
then delete the cruft left at the start and end of the file:
:1,/^begin/-1 d
:$?^end?+1,$ d
Note the use of $?^end?+1 to search backwards from the end of the file to find the last line starting with "end".
N.B. the last two will delete too much if the file starts with "begin" or ends with "end" at that point, so check before you use them.