SI-6406 Restore deprecated API

The original patch for SI-6406 was intended for 2.10 but during
those volatile weeks of early autumn, it missed the boat.

A deprecated method was incorrectly tagged at 2.10 and later
removed; this restores the method and its test, and resets
the deprecation clock to 2.11.

The deprecation tool should confirm that changes occur on the
git timeline as claimed.
Merge pull request #2519 from som-snytt/issue/regex-revert

SI-6406 Restore deprecated API