git grep -P is multiline for negative lookahead/behind

Negative lookahead/lookbehind with `git grep -P` considers the surrounding lines of an otherwise positive match. This differs from `grep -P` behavior. It does not repro for *positive* lookahead/lookbehind.

Example: $ echo -e 'Bar\nBar Baz\nBat' > test.txt && git add test.txt $ git grep -P 'Bar(?!\sB)' test.txt # Find 'Bar' not followed by '\sB'

With regular grep, this works:

$ grep -P 'Bar(?!\sB)' test.txt Bar

Because \s includes \n, the negative lookahead unexpectedly finds the B on line 2, and therefore line 1 fails to match the full expression.

Positive lookahead works like `grep -P`, in that it isn't unexpectedly multiline:

$ git grep -P 'Bar(?=\sB)' test.txt # Find 'Bar' followed by '\sB' test.txt:Bar Baz $ grep -P 'Bar(?=\sB)' test.txt Bar Baz

Is this expected behavior, and if so, why/where is this documented?

git: git version 2.8.0.rc3.226.g39d4020, also 2.9.2.517.gf8f7adc, both with libpcre grep: grep (GNU grep) 2.16