> Using #include "_sre.c" in _sre.c looks weird. Instead of huge sections delimited by "#ifdef SRE_RECURSIVE", I would prefer something similar to the stringlib. ".h" template files included more than once. I also expect shorter files: _sre.c is close to 4000 lines of C code :-(

Agree, but a patch will be larger and harder for the synchronization and for the review in Rietveld. I'm going first solve other issues (issue18647, issue18672) before creating a large final patch.

> The value of SIZEOF_SRE_CHAR looks suspicious.

Good catch. Actually this macro is used only in order to skip some checks for UCS4. It should not affects the correctness, only possible the performance.

> Does test_re have some non-ASCII tests? If not, we should probably start by adding such tests!

There is a small number (about 10) of tests for non-ASCII data.