Python RegEx through the ages, Appendix C: Summary table

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Colliding names used by multiple non-interchangable modules (regex and pcre) are preceded by a disambiguating numerical superscript.

Included with	Base API	Engine	Main module name	C module name	Other modules	Notes
0.9.0–0.9.1	First (`exec`)	Henry Spencer	`regexp`	`regexp`		Posix Extended format. First public release of Python. Very simple API.
0.9.2–0.9.4	First (`match`)	Henry Spencer	`regexp`	`regexp`		`exec` method renamed to `match`.
0.9.5	Second	GNU	^(1st)`regex`	^(1st)`regex`	`regex_syntax`	Emacs-style format by default, but changeable with flags.
0.9.6–2.4	Second	Ylönen based	^(1st)`regex`	^(1st)`regex`	`regex_syntax`, later also `regsub`	Changed to engine written by Tatu Ylönen. Emitting DeprecationWarning since 2.1, finally removed in 2.5.0alpha1.
0.9.5–1.4	First (`match`)	(same as ^(1st)`regex`)	`regexp`	^(1st)`regex`		Wrapper around the contemporary ^(1st)`regex` module. Module removed in 1.5.0beta1.
1.5.0	Third	Ylönen based (bytecode), custom (parsing)	`re` or `re1`	`reop`		Perl-style format. New thread-safe API defined. Introduced as `re` in 1.5.0alpha3, implementation never fully debugged before it was superseded in 1.5.0alpha4. Retained as `re1` for the 1.5.0 release, removed before 1.5.1.
1.5–2.3	Third	PCRE (old)	`re` or `pre`	^(1st)`pcre`		Introduced in 1.5.0alpha4 using what was then (late 1990s) a contemporary PCRE library. In particular, Unicode regular expressions were not yet supported. Renamed to `pre` in Python 2.^† Removed in Python 2.4.
1.6^†–present	Third	SecretLabs based	`sre` or `re`	`_sre`	`sre_compile`, `sre_parse`, `sre_constants`	Introduced with Python 2^† as `sre` due to the need for a `re` implementation that supported Unicode regular expressions. Initially used a recursive matching scheme, this was changed in Python 2.4.0alpha1. Importing as `sre` deprecated in Python 2.5, only availabile as `re` in Python 3.x.
PyPI `python-pcre`	mostly Third	PCRE	^(2nd)`pcre`	`_pcre`		Extension binding by Arkadiusz Wahlig. Uses the third API except for substitution format string syntax, and can be configured to use the third-API syntax there too. Lacks the scanner APIs and DEBUG and LOCALE flags of the third API. Current PCRE does support Unicode regular expressions; note the name collision.
Planned for future inclusion; PyPI `regex`	Third with extensions	SecretLabs based	^(2nd)`regex`	`_regex`	`_regex_core`	Extension by Matthew Barnett, forked from `re` (`sre`). Prominent enough to get a mention in the documentation and “in principle approval for eventual stdlib inclusion”. Note the name collision with the second API. Backward-incompatibile behaviour fixes behind a version switch selectable in regex syntax and in API.

† Python 1.6 is basically the state of Python 2 at the point that Guido left CNRI, released under contractual obligation or something similar: it incorporates many but not all distinctively 2.x features, it is accordingly not a true Python 1.x release; hence, “What’s new in Python 2.0” compares it with Python 1.5.2.