Is there a function that returns index where RegEx match starts?

For multiple matches you can use code similar to this:

Regex rx = new Regex("as");
foreach (Match match in rx.Matches("as as as as"))
{
    int i = match.Index;
}

Instead of using IsMatch, use the Matches method. This will return a MatchCollection, which contains a number of Match objects. These have a property Index.

Use Match instead of IsMatch:

    Match match = Regex.Match("abcde", "c");
    if (match.Success)
    {
        int index = match.Index;
        Console.WriteLine("Index of match: " + index);
    }

Output:

Index of match: 2