test: RegexMatchingTest

[alxkm]

Added javadocs.
Added RegexMatchingTest.

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized it.
• All filenames are in PascalCase.
• All functions and variable names follow Java naming conventions.
• All new algorithms have a URL in their comments that points to Wikipedia or other similar explanations.
• All new code is formatted with clang-format -i --style=file path/to/your/file.java

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 48.73%. Comparing base (4374a50) to head (eea5878).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5403      +/-   ##
============================================
+ Coverage     48.16%   48.73%   +0.56%     
- Complexity     2975     3027      +52     
============================================
  Files           524      524              
  Lines         15273    15266       -7     
  Branches       2901     2901              
============================================
+ Hits           7357     7440      +83     
+ Misses         7610     7518      -92     
- Partials        306      308       +2     

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[siriak]

Please merge this and other PRs by yourself once they are approved, thanks!