Remove everything except regex match in Vim

Question

My specific case is a text document that contains lots of text and IPv4 addresses. I want to remove everything except for the IP addresses.

I can use :vglobal<

Answer 1

This effect can be accomplished by using sub-replace-special substitution and setreg() linewise

:let @a=""
:%s//\=setreg('A', submatch(0), 'l')/g
:%d _
:pu a
:0d _

or all in one line as such:

:let @a=""|%s//\=setreg('A', submatch(0), 'l')/g|%d _|pu a|0d _

Overview: Using a substitution to append each match into register "a" linewise then replace the entire buffer with the contents of register "a"

Explanation:

1. let @a="" empty the "a" register that we will be appending into
2. %s//\=setreg('A', submatch(0), 'l')/g substitute globally using the last pattern
3. the \=expr will replace the pattern with the contents of the expression
4. submatch(0) get the entire string of what just matched
5. setreg('A', submatch(0), 'l') append (note: the capital "a") to @a the matched string, but linewise
6. %d _ delete every line into the black hole register (aka @_)
7. pu a put the contents of @a into the buffer
8. 0d _ delete the first line

Concerns:

• This will trash one of your registers. This example trashed @a
• Uses the last search pattern. Although you can modify the substitute command with whatever pattern you want: %s//\=setreg('A', submatch(0), 'l')/g

For more help

:h :s\=
:h :let-@
:h submatch()
:h setreg()
:h :d
:h :p