Parsing multi-line string to extract only matched lines

Question

I have a multi-line string and I'd like to only extract lines that match a certain pattern and discard the rest of the string.

Original string:

lorem ipsum dolor sit amet consectetur
nunc KEEP THIS LINE ut massa lorem
DO NOT KEEP THIS LINE aenean blandit
nunc KEEP THIS LINE et justo quis
praesent at velit felis vel

Desired final string:

KEEP THIS LINE ut massa lorem
KEEP THIS LINE et justo quis

The regex I have so far is:

(?ms).*?(?-s)^nunc (KEEP THIS LINE.*?$)

I can just replace the match with the backreference \1. However, this fails to replace from the end of the last matched pattern to the end of the string.

The regex:

(?ms).*?(?-s)(^nunc (KEEP THIS LINE.*?$)|(?s:).*?\Z)

successfully identifies the pattern between the last desired line and the end of the string, but I can't figure out how to remove it.

Answer 1

Replacing ^(?:nunc (KEEP THIS LINE.*)|.*\n)$ with \1 will achieve the desired output:

• Due to the alternator (|), the expression ^(?:nunc (KEEP THIS LINE.*)?|.*\n)$ will forcefully match the entire line, without capturing it (?: designates a non-capturing group).

• If the line begins with nunc KEEP THIS LINE, \1 will refer to the insides of the parentheses, i.e., the string beginning with KEEP THIS LINE and ending at the end of the line.

• If the line does not begins with nunc KEEP THIS LINE, .*\n will match the entire line, including the terminating newline character.

The above regular expression works fine with Perl.

In Notepad++ with Windows line breaks, you can use this one instead:

^(?:nunc (KEEP THIS LINE.*)|.*(\r\n)?)

Note that this will only remove the last line, not remove it.