It's a grammar that makes heavy use of regular expressions. So make sure you brush up your skill on backward/forward references and lookarounds, because all these can be used.

old_log_entries = old_log.split /(?=commit [0-9a-f]{40})/ # Lookahead assertions FTW

old_log.split /(?=commit [0-9a-f]{40})/ # Lookahead assertions FTW

I'm using a negative lookahead to mean "ignore all in /node_modules/ unless it's that package" using