fix(regex): Construct backreferences to ensure entity consistency

[effigies]

The regexes currently generated by bidsschematools were overly permissive, allowing mismatches between the entities in the directory and the entities in the filename. This was caught because the Python validator moved to depending on bidsschematools, and pybids has an additional battery of checks. (See bids-standard/pybids#1080.)

I've copied the failing checks in here as well as run pybids' test using this branch of bidsschematools.

Really, we shouldn't be using regexes, but doing it right is its own challenge and this fixes a known issue. Further, this does not handle the case where you might have, e.g., ses-siteA_dwi.json that applies to one session across all subjects.

For my future reference, to properly implement inheritance in regexes, we would need:

(?:sub-(?P<subject>[0-9a-zA-Z]+)/)? # Match subject dir
(?(subject)  # If subject dir
    (?:ses-(?P<sessiondirlabel>[0-9a-zA-Z]+)/)?  # Permit session dir
    (?:(?P<datatype>dwi)/)?  # Permit datatype dir
    sub-(?P=subject)_  # Require subject entity
    (?(sessiondirlabel)  # If session dir
        ses-  # Require start of session entity
    )
| # Else
    (?P<root>ses-)?  # Permit subject entity in root directory
)
(?P<session>  # Capture session label
    (?(sessiondirlabel)  # If session directory was matched
        (?P=sessiondirlabel)  # Match session directory label
    |
        (?(root)[0-9a-zA-Z]+)  # Otherwise match any session label
    )
)
(?(sessiondirlabel)_)(?(root)_)  # Match _ if session entity
(?:acq-(?P<acquisition>[0-9a-zA-Z]+)_)?  # Match acquisition entity
# and so on
(?P<suffix>dwi)
(?P<extension>\.bvec|\.bvec|\.json)

Figuring this out was enough of a task. Programmatically generating it doesn't seem worth it right now.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.22%. Comparing base (0e506f0) to head (e31e32a).
Report is 296 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1889      +/-   ##
==========================================
+ Coverage   88.06%   88.22%   +0.16%     
==========================================
  Files          16       16              
  Lines        1391     1393       +2     
==========================================
+ Hits         1225     1229       +4     
+ Misses        166      164       -2     

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