REGEXP_FILE
  RegExpPatternImpl: <([ab]+)=\2>
    RegExpBranchImpl: <([ab]+)=\2>
      RegExpGroupImpl: <([ab]+)>
        PsiElement(GROUP_BEGIN)('(')
        RegExpPatternImpl: <[ab]+>
          RegExpBranchImpl: <[ab]+>
            RegExpClosureImpl: <[ab]+>
              RegExpClassImpl: <[ab]>
                PsiElement(CLASS_BEGIN)('[')
                RegExpCharImpl: <a>
                  PsiElement(CHARACTER)('a')
                RegExpCharImpl: <b>
                  PsiElement(CHARACTER)('b')
                PsiElement(CLASS_END)(']')
              RegExpQuantifierImpl: <+>
                PsiElement(PLUS)('+')
        PsiElement(GROUP_END)(')')
      RegExpCharImpl: <=>
        PsiElement(CHARACTER)('=')
      RegExpBackrefImpl: <\2>
        PsiElement(BACKREF)('\2')