How do search this in reg expression [0-9] x [0-9]

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regular-expression

C#6.0

How do search this in reg expression [0-9] x [0-9] || [0-9]x[0-9].

My goal here is to search and verify if the statement is a multiplication statement. I'm not really good in regular expression.

public void CheckMultiplicationEntity()
        {
            int lineno = 0;
            bool start = false;
            foreach (string line in _contentList)
            {
                lineno++;
                if (line.Contains("<text>"))
                {
                    start = true;
                }
                if (start)
                {
                    foreach (Match match in Regex.Matches(line, @"([0-9]x[0-9]|[0-9\sx\s0-9]"))
                    {
                        List<ErrorModel> errorlist = ErrorList;
                        ErrorModel errorModel = new ErrorModel()
                        {
                            LineNumber = lineno,
                            ErrorMessage = "Please check \"x\" should ×",
                            Text = ShortenText(match.Value)
                        };
                        errorlist.Add(errorModel);
                    }
                }
            }
}