Grammar to vaidate a re2 regular expression in compliance with: https://github.com/google/re2/wiki/Syntax/.
Test patterns taken from: https://github.com/golang/go/tree/master/src/regexp/testdata.
To test:
$ antlr4 Re2.g4 -o out && cd out && javac Re2*.java && java org.antlr.v4.gui.TestRig Re2 test_root ../tests/patterns.txt && cd ..