Capture a specific line if it exists with regex - regex

Question

If I have files that could either be:
numbers:
32
45
999
56
or
numbers:
23
45
56
The 999 is a constant, but the other numbers & number of lines change.
Is there a way of capturing:
above 999 followed by an empty line in the first case (i.e. excluding 999)
above the empty line in the second case, where 999 doesn't exist
So far I've tried:
(numbers:(?:\n.+)*)(\n999) — this works great in the first case; the first group captures everything above 999. It obv doesn't work where there's no 999, so...
(numbers:(?:\n.+)*)(\n999)? — I would have thought this would work for both cases. But in the first case, this captures the 999 in the first group, I guess because it's greedy and the ? makes the (\n999) optional, so the first group is free to capture it.
It's also possible I'm massively overcomplicating this and there's some easy solution.
Thanks a lot!

Answer 1

Here is a regex that does it without using lookahead:
^(numbers:(?:\n.+)*?)(?:\n(?:999)?$)
RegEx Demo
RegEx Details:
^: Start line
(: Start capture group #1
numbers:: Match numbers: text
(?:\n.+)*?: Match a line break followed by 1+ character in a line. Repeat this 0 or more times (non-greedy)
): End capture group #1
(?:\n(?:999)?$): Must be followed by a line break and 999 or empty line

Answer 2

Use
(numbers:(?:\n(?!999).+)*)(\n999)?
See regex proof.
The question mark was the right move. The (?!999) negative lookahead is restricting the (.+)* and it does not cross optional nines.
If you do not want cross another numbers add to lookahead:
(numbers:(?:\n(?!999|numbers:).+)*)(\n999)?
EXPLANATION
--------------------------------------------------------------------------------
( group and capture to \1:
--------------------------------------------------------------------------------
numbers: 'numbers:'
--------------------------------------------------------------------------------
(?: group, but do not capture (0 or more
times (matching the most amount
possible)):
--------------------------------------------------------------------------------
\n '\n' (newline)
--------------------------------------------------------------------------------
(?! look ahead to see if there is not:
--------------------------------------------------------------------------------
999 '999'
--------------------------------------------------------------------------------
) end of look-ahead
--------------------------------------------------------------------------------
.+ any character except \n (1 or more
times (matching the most amount
possible))
--------------------------------------------------------------------------------
)* end of grouping
--------------------------------------------------------------------------------
) end of \1
--------------------------------------------------------------------------------
( group and capture to \2 (optional
(matching the most amount possible)):
--------------------------------------------------------------------------------
\n '\n' (newline)
--------------------------------------------------------------------------------
999 '999'
--------------------------------------------------------------------------------
)? end of \2 (NOTE: because you are using a
quantifier on this capture, only the LAST
repetition of the captured pattern will be
stored in \2)
You prefer no lookarounds? Here:
^(numbers:[\s\S]*?)\n(?:999)?$
See proof.
EXPLANATION
--------------------------------------------------------------------------------
^ the beginning of the string
--------------------------------------------------------------------------------
( group and capture to \1:
--------------------------------------------------------------------------------
numbers: 'numbers:'
--------------------------------------------------------------------------------
[\s\S]*? any character of: whitespace (\n, \r,
\t, \f, and " "), non-whitespace (all
but \n, \r, \t, \f, and " ") (0 or more
times (matching the least amount
possible))
--------------------------------------------------------------------------------
) end of \1
--------------------------------------------------------------------------------
\n '\n' (newline)
--------------------------------------------------------------------------------
(?: group, but do not capture (optional
(matching the most amount possible)):
--------------------------------------------------------------------------------
999 '999'
--------------------------------------------------------------------------------
)? end of grouping
--------------------------------------------------------------------------------
$ before an optional \n, and the end of the
string