REGEXP_FILE
  RegExpPatternImpl: <abc\u>
    RegExpBranchImpl: <abc\u>
      RegExpCharImpl: <a>
        PsiElement(CHARACTER)('a')
      RegExpCharImpl: <b>
        PsiElement(CHARACTER)('b')
      RegExpCharImpl: <c>
        PsiElement(CHARACTER)('c')
      RegExpCharImpl: <\u>
        PsiElement(INVALID_UNICODE_ESCAPE_TOKEN)('\u')