Python 'regex' module support

[OnlineCop]

This may be a duplicate of #1219.

I see multiple issues being opened regarding Python syntax: #1271, #1403, #1404.

Should we offer users who choose Python as the flavor, a choice to select either the re or the regex modules?

regex is backward-compatible with re but extends many of the PCRE features, like inline flags, \K, recursion, etc.

[firasdib]

Wouldn't their need be filled by simply using the PCRE flavor?

[OnlineCop]

Mostly, yes, but if PCRE were selected as the flavor, the Code Generator is using the re module instead of the regex module when you change to the Python language.

Most of those advanced features are supported under the regex module, so we might want to change re for regex when the Flavor is PCRE and the Code Generator language is set to Python.

[Doqnach]

Possible alternative is to have multiple code generators for Python? (2 vs 3, re vs regex)

[genericusername3]

Wouldn't their need be filled by simply using the PCRE flavor?

No, actually. In contrast to PCRE, the regex module supports variable-width lookbehinds, for instance (like ECMAScript, but the named group structure in ECMA is (?<name>pattern) instead of (?P<name>pattern))