REGEXP_FILE
  RegExpPatternImpl: <a>
    RegExpBranchImpl: <a>
      RegExpCharImpl: <a>
        PsiElement(CHARACTER)('a')
  PsiErrorElement:Unmatched closing ')'
    <empty list>
  PsiElement(GROUP_END)(')')
  RegExpPatternImpl: <b>
    RegExpBranchImpl: <b>
      RegExpCharImpl: <b>
        PsiElement(CHARACTER)('b')