title: Find how many characters a regex has looked at in a string before succeeding/failing
body: <p>I want to see how many characters the regex has looked at before it succeeds or fails (to support incrementally re-lexing something). I haven't noticed a direct way to do this in the <a href="https://github.com/rust-lang/regex" rel="nofollow noreferrer">Rust regex library's</a> API.</p>
  
  <p>If I could somehow intercept the <code>&amp;str</code> I give to library, I could record the furthest character it's had to observe. I'm not sure how to do this, as I am new to Rust. If this were Java, I would use a proxy/implement <code>CharSequence</code>.</p>
  
  <p>Bonus points if it works with <a href="https://docs.rs/regex/1.1.0/regex/struct.RegexSet.html" rel="nofollow noreferrer"><code>RegexSet</code></a>.</p>