REGEXP_FILE
  RegExpPatternImpl: <a[bc]d>
    RegExpBranchImpl: <a[bc]d>
      RegExpCharImpl: <a>
        PsiElement(CHARACTER)('a')
      RegExpClassImpl: <[bc]>
        PsiElement(CLASS_BEGIN)('[')
        RegExpCharImpl: <b>
          PsiElement(CHARACTER)('b')
        RegExpCharImpl: <c>
          PsiElement(CHARACTER)('c')
        PsiElement(CLASS_END)(']')
      RegExpCharImpl: <d>
        PsiElement(CHARACTER)('d')