REGEXP_FILE
  RegExpPatternImpl: <()ef>
    RegExpBranchImpl: <()ef>
      RegExpGroupImpl: <()>
        PsiElement(GROUP_BEGIN)('(')
        RegExpPatternImpl: <>
          RegExpBranchImpl: <>
            <empty list>
        PsiElement(GROUP_END)(')')
      RegExpCharImpl: <e>
        PsiElement(CHARACTER)('e')
      RegExpCharImpl: <f>
        PsiElement(CHARACTER)('f')