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