Question

I use this code to check if a string exist in a text file that I loaded into memory

foreach (Match m in Regex.Matches(haystack, needle))
    richTextBox1.Text += "\nFound @ " + m.Index;

The regex returns the positions where a match occurred but I want
to know the line number?

Answer 1

The best solution would be to call a method that gets the line number only if a match occurs.
This way the performance is not much affected if multiple files were checked and the regexp with \n will work. Found this method somewhere on stackoverflow:

    public int LineFromPos(string input, int indexPosition)
    {
        int lineNumber = 1;
        for (int i = 0; i < indexPosition; i++)
        {
            if (input[i] == '\n') lineNumber++;
        }
        return lineNumber;
    }