REGEXP_FILE
  RegExpPatternImpl: <([ab]+=\1)>
    RegExpBranchImpl: <([ab]+=\1)>
      RegExpGroupImpl: <([ab]+=\1)>
        PsiElement(GROUP_BEGIN)('(')
        RegExpPatternImpl: <[ab]+=\1>
          RegExpBranchImpl: <[ab]+=\1>
            RegExpClosureImpl: <[ab]+>
              RegExpClassImpl: <[ab]>
                PsiElement(CLASS_BEGIN)('[')
                RegExpCharImpl: <a>
                  PsiElement(CHARACTER)('a')
                RegExpCharImpl: <b>
                  PsiElement(CHARACTER)('b')
                PsiElement(CLASS_END)(']')
              RegExpQuantifierImpl: <+>
                PsiElement(PLUS)('+')
            RegExpCharImpl: <=>
              PsiElement(CHARACTER)('=')
            RegExpBackrefImpl: <\1>
              PsiElement(BACKREF)('\1')
        PsiElement(GROUP_END)(')')