Use POSIX ERE for sub function

[DavyCats]

Motivation

This PR implements openwdl/wdl#243

Approach

Uses the POSIX implementation of the regex package.

Checklist

• Add appropriate test(s) to the automatic suite
• Use make pretty to reformat the code with black
• Use make check to statically check the code using Pyre and Pylint
• Send PR from a dedicated branch without unrelated edits
• Ensure compatibility with this project's MIT license

[coveralls]

Pull Request Test Coverage Report for Build 1487

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 96.804%

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Totals
Change from base Build 1483:	0.0%
Covered Lines:	4846
Relevant Lines:	5006

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💛 - Coveralls

[DavyCats]

There was an error in the linting, because there was no stub for the regex module. I added one, though I'm not 100% sure this is the correct way of doing this. Please let me know if something is amiss.

[mlin]

@DavyCats thx & sorry for slow uptake on this, I've been tied up moving apartments. Will help with the hygiene obstacles

[DavyCats]

@mlin No problem, there's no haste behind this whatsoever, so if you don't have the time right now it can wait.