Add test regular expression.

[marcofavorito]

Types of changes

What types of changes does your code introduce? Put an x in all the boxes that apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

[marcofavorito]

we need these and the same for the other concrete visitors because now they are pure virtual methods in the abstract class, for better consistency.

[codecov]

Codecov Report

Merging #42 into develop will increase coverage by 0.17%.
The diff coverage is 35.22%.

@@             Coverage Diff             @@
##           develop      #42      +/-   ##
===========================================
+ Coverage    33.53%   33.70%   +0.17%     
===========================================
  Files           42       42              
  Lines         1846     1985     +139     
  Branches      1165     1208      +43     
===========================================
+ Hits           619      669      +50     
- Misses         281      336      +55     
- Partials       946      980      +34     

Flag	Coverage Δ
#unittests	33.70% <35.22%> (+0.17%)	⬆️

Impacted Files	Coverage Δ
lib/include/basic.hpp	87.50% <ø> (ø)
lib/include/delta.hpp	21.42% <0.00%> (-38.58%)	⬇️
lib/include/propositional_logic.hpp	40.90% <0.00%> (-49.10%)	⬇️
lib/include/types.hpp	50.00% <ø> (ø)
lib/include/utils/print.hpp	7.14% <0.00%> (-92.86%)	⬇️
lib/include/visitor.hpp	100.00% <ø> (+93.33%)	⬆️
lib/src/delta.cpp	30.35% <9.09%> (-5.20%)	⬇️
lib/src/logic.cpp	42.46% <20.00%> (-2.85%)	⬇️
lib/test/src/test_translate.cpp	13.26% <22.58%> (+1.75%)	⬆️
lib/include/nnf.hpp	46.42% <46.15%> (-3.58%)	⬇️
... and 5 more

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[francescofuggitti]

Great!! LGTM, thanks!