REGEXP_FILE
  RegExpPatternImpl: <[a[^b]]>
    RegExpBranchImpl: <[a[^b]]>
      RegExpClassImpl: <[a[^b]]>
        PsiElement(CLASS_BEGIN)('[')
        RegExpCharImpl: <a>
          PsiElement(CHARACTER)('a')
        RegExpClassImpl: <[^b]>
          PsiElement(CLASS_BEGIN)('[')
          PsiElement(CARET)('^')
          RegExpCharImpl: <b>
            PsiElement(CHARACTER)('b')
          PsiElement(CLASS_END)(']')
        PsiElement(CLASS_END)(']')