@Override
  public void visitRegExpBackref(final RegExpBackref backref) {
    final RegExpGroup group = backref.resolve();
    if (group == null) {
      myHolder.newAnnotation(HighlightSeverity.ERROR, RegExpBundle.message("error.unresolved.back.reference"))
      .highlightType(ProblemHighlightType.LIKE_UNKNOWN_SYMBOL).create();
    }
    else if (PsiTreeUtil.isAncestor(group, backref, true)) {
      myHolder.newAnnotation(HighlightSeverity.WARNING,
                             RegExpBundle.message("error.back.reference.is.nested.into.the.capturing.group.it.refers.to")).create();
    }
  }
@Override
  public void visitRegExpBackref(final RegExpBackref backref)