Question

I want to retrieve a number from a string wherever the number starts with 8,9 or 6 and length of the number should be 8 OR 9 Characters. E.g 92000000,9200 0000,9200-0000.

Answer 1

How about this: (?<!\d)([896]\d{3})(?:[-\s]?)(\d{4})(?!\d).

The (?:[-\s]?) eats the optional delimiters space or dash as a non-capturing group.

You get your number by concatenating the match groups 1 and 2:

var input = new string[] {
    "81000000", "92000000", "9200 0000", "9200-0000"
};
var regex = new Regex (@"(?<!\d)([896]\d{3})(?:[-\s]?)(\d{4})(?!\d)");
foreach (var str in input) {
    var match = regex.Match (str);
    Console.WriteLine ("TEST: {0} {1} - {2}", str, match.Success,
                       match.Groups [1].Value + match.Groups [2].Value);
}

I have also tried (?<!\d)([896]\d{3}(?:[-\s]?)\d{4})(?!\d) and that won’t remove the delimiter character from the match result.