Switch from re to 3rd-party regex module

[Seirdy]

Switch to the more featureful, API-compatible regex module. This module
is recommended for more advanced regular expressions in Python's own
documentation: https://docs.python.org/3/library/re.html

Regex patterns can now use features such as variable-length lookarounds,
lookarounds in conditional patterns, recursive patterns, fuzzy matching,
\m and \M for the start/end of a word, and many others. See
https://pypi.org/project/regex/ for more information.

[Seirdy]

My rationale for introducing a third-party dependency like this is that none of the existing find-replace solutions I’ve tried support anything close to the advanced featureset of regex. On the commandline, between https://github.com/chmln/sd, ripgrep with rg --engine=pcre2 --passthru <find> -r <replace>, and some others, subst could become the most advanced CLI for regex substitution out there just by changing import re to import regex as re.

[msztolcman]

First: thanks for an idea and PR!
I find the idea great, and would be also great to merge it, but there are usecases where installing dependencies out of stdlib are a pain. However, most of the ones I know, relates to install subst without pip, just by download raw file and us it as is.

Please, handle this situation (should be enough to try import regex, and if fails - use re instead).
Also - please update pipenv dependencies. Should be enough to initialize virtualenv (pipenv install --dev), then install new dependency (pipenv install regex).

Would you like to update your PR for this stuff?