The fastest way to count string occurences in another string in c#

Question

I have to count string occurences in one very loooong string (about 30mb in plain text) I use the following code for now: int count = new Regex(Regex.Escape(stringThatIamLookingFor)).Matches(stringToSearchIn).Count; but is is too slow. It takes about 3 minutes on i7 and 16gb ram. The example data is:

43.996442,-31.768039
43.996432,-31.768039
43.996432,-31.768049
43.996422,-31.768049
43.996422,-31.768059

I want to count (for example) .7 Is there faster way than regeex?

ok, solved

The fastest function so far is: (I need to check only two chars.)

public int countOccurences2(string source, char charOne, char charTwo)
    {
        int count = 0;
        for (int i=0; i<=source.Length-1; i=i+2)
            if (source[i] == charOne && source[i + 1] == charTwo) { count++; }
        return count;
    }

Answer 1

from this question: How would you count occurrences of a string within a string?

the following code seems the most performing:

int count = 0, n = 0;

if(substring != "")
{
    while ((n = source.IndexOf(substring, n, StringComparison.InvariantCulture)) != -1)
    {
        n += substring.Length;
        ++count;
    }
}

solution provided by Richard Watson in the mentioned question