The key here is that both -o and --color will force grep to fine every match on each matching line. And it looks like that triggers the buggy code path. Without

The key here is that both -o and --color will force grep to fine every match on each matching line. And it looks like that triggers the buggy code path. Without those options, it's possible grep won't need to find each match on each matching line, but rather, just whether a line matches at all.