Yeah. I haven't tried out your code, but an API idea might be to have the caller instruct the iterator what to do with any improper/undelimited record at the end -- includeUnterminated=False or something. Only the caller really knows what is desired.
One observation, algo performance-wise, is that the fastest substring searches are usually achieved by using a fast libc SSE/AVX vectorized `memchr` to find the _least likely_ character in the substring. Sometimes one knows this from vague properties of the distribution. For example, framing the email messages in UUCP-style mailbox with '^From ' kinds of delimiting, `memchr` for the capital 'F' skips over most text most of the time. This idea may not work in a corpus of emails in ALL CAPS ALL THE TIME, but such a corpus should be rare. Anyway, this idea is just sort of the simplest/first step into a wider world of efficient substring search algorithms that you might enjoy reading about some day.
