REGEXP_FILE
  RegExpPatternImpl: <(c[ab]?)b>
    RegExpBranchImpl: <(c[ab]?)b>
      RegExpGroupImpl: <(c[ab]?)>
        PsiElement(GROUP_BEGIN)('(')
        RegExpPatternImpl: <c[ab]?>
          RegExpBranchImpl: <c[ab]?>
            RegExpCharImpl: <c>
              PsiElement(CHARACTER)('c')
            RegExpClosureImpl: <[ab]?>
              RegExpClassImpl: <[ab]>
                PsiElement(CLASS_BEGIN)('[')
                RegExpCharImpl: <a>
                  PsiElement(CHARACTER)('a')
                RegExpCharImpl: <b>
                  PsiElement(CHARACTER)('b')
                PsiElement(CLASS_END)(']')
              RegExpQuantifierImpl: <?>
                PsiElement(QUEST)('?')
        PsiElement(GROUP_END)(')')
      RegExpCharImpl: <b>
        PsiElement(CHARACTER)('b')