Proposed finalization note:
The design of this SRFI intends to allow implementors to compile SREs to their native regexp libraries.
As such, the submatch semantics are left unspecified. The preferred semantics is leftmost-longest,
i.e. if multiple submatches are possible the leftmost is preferred, choosing the longest submatch as the
tie-breaker among those starting at the same position. The reference implementation, IrRegex, and the
cited Re implementations provide this semantics, however most backtracking algorithms only approximate
this behavior. Similarly, the non-greedy-* patterns should provide leftmost-shortest semantics.