fix issue with non-required regex-matched columns

[cosmicBboy]

Fixes #1248

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (4d16290) 93.73% compared to head (b00c2b0) 93.74%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1251   +/-   ##
=======================================
  Coverage   93.73%   93.74%           
=======================================
  Files          90       90           
  Lines        6708     6711    +3     
=======================================
+ Hits         6288     6291    +3     
  Misses        420      420           

Impacted Files	Coverage Δ
pandera/backends/pandas/base.py	100.00% <100.00%> (ø)
pandera/backends/pandas/container.py	98.55% <100.00%> (+0.01%)	⬆️

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