REGEXP_FILE
  RegExpPatternImpl: <a[]b>
    RegExpBranchImpl: <a[]b>
      RegExpCharImpl: <a>
        PsiElement(CHARACTER)('a')
      RegExpClassImpl: <[]b>
        PsiElement(CLASS_BEGIN)('[')
        RegExpCharImpl: <]>
          PsiElement(CHARACTER)(']')
        RegExpCharImpl: <b>
          PsiElement(CHARACTER)('b')
        PsiErrorElement:Unclosed character class
          <empty list>