Vendor things

third-party/vendor/regex/testdata/README.md

@@ -0,0 +1,22 @@
+This directory contains a large suite of regex tests defined in a TOML format.
+They are used to drive tests in `tests/lib.rs`, `regex-automata/tests/lib.rs`
+and `regex-lite/tests/lib.rs`.
+
+See the [`regex-test`][regex-test] crate documentation for an explanation of
+the format and how it generates tests.
+
+The basic idea here is that we have many different regex engines but generally
+one set of tests. We want to be able to run those tests (or most of them) on
+every engine. Prior to `regex 1.9`, we used to do this with a hodge podge soup
+of macros and a different test executable for each engine. It overall took a
+longer time to compile, was harder to maintain and it made the test definitions
+themselves less clear.
+
+In `regex 1.9`, when we moved over to `regex-automata`, the situation got a lot
+worse because of an increase in the number of engines. So I devised an engine
+independent format for testing regex patterns and their semantics.
+
+Note: the naming scheme used in these tests isn't terribly consistent. It would
+be great to fix that.
+
+[regex-test]: https://docs.rs/regex-test