CPython uses (possibly patched) PCRE library http://www.pcre.org/ (BSD license). Nothing would preclude its usage in MicroPython, but:

1. Worst-case performance is awful.
2. Nobody would ever say PCRE is optimized for code size/memory usage.

So, PCRE would definitely fit "unix" port, to be completely compatible with CPython "re" module. But I'm not sure how usable it will be for MCU ports.

So, what I imagine is be able to support 2 (or more?) regex libraries - one PCRE for complete compatibility, another using other library(ies) for unbloatness, at the expense of reduced feature set. Note that "reduced" subset can live in a separate module (say, "remini").

This ticket would be a good place to discuss that and collect candidates for alternative re lib.