Previously, attempting to split around a delimiter would fail if that delimiter was preceded by any positive length match.

In my case, I was trying to use split to consume blank delimiters, but yield quote delimiters. I ended up writing a test like:

 assert_eq!(
        Regex::new(r#"(?=")|(?<=")| +"#)
            .unwrap()
            .split(" a\"b")
            .collect::<Vec<_>>(),
        vec!["", "a", "\"", "b"]
    );

But it was failing to split on the zero-width position before the quote:

thread 'option::test_onig' panicked at 'assertion failed: `(left == right)`
  left: `["", "a\"", "b"]`,
 right: `["", "a", "\"", "b"]`', src/option.rs:195:5

With this change, my test now passes. What do you think of it?