using '^' (or '\A') with multi-line search doesn't always produce expected result #1878

Initially found on HN: https://news.ycombinator.com/item?id=27324265

Specifically, while this output is correct (since ^ is set to be in regex multi-line mode always):

$ printf 'a\nbaz\nabc\n' | rg -U '^b'
baz

It should be the case that using (?-m)^b or \Ab would not print baz as a match. But that's not the case here:

$ printf 'a\nbaz\nabc\n' | rg -U '(?-m)^b'
baz
$ printf 'a\nbaz\nabc\n' | rg -U '\Ab'
baz

The issue here is that in this case, ripgrep isn't memory mapping the input. In that case, ripgrep tries to be "smart" and not actually read the entire contents on to the heap if it knows the pattern can't match through a line terminator. But in this case, we can't quite make that assumption since anchors can match line terminators as look-around.