Variables are leaking from my methods in C#

Question

I have currently have a block of code that is repeated throughout my program inside Button Clicks with the exception of a few variables. The issue is that when I go from clicking one button to another button, plHTML starts leaking text from the previous button. This did not happen before I tried making the method.

How can I prevent the variables from leaking into each other?

This is the method I have attempted to make

    String cleanCombo1;
    String cleanCombo2;
    String cleanCombo3;
    String cleanCombo4;
    String cleanCombo;
    public string GetTextBetween(string firstPart, string secondPart, string lastPart)
    {
        String St1 = plHTMLP1.Text;
        int pFrom1 = St1.IndexOf(firstPart) + firstPart.Length;
        int pTo1 = St1.IndexOf(lastPart, pFrom1);
        if (St1.Substring(pFrom1, pTo1 - pFrom1).Contains(secondPart))
        {
            cleanCombo1 = St1.Substring(pFrom1, pTo1 - pFrom1);
        }
        String St2 = plHTMLP2.Text;
        int pFrom2 = St2.IndexOf(firstPart) + firstPart.Length;
        int pTo2 = St2.IndexOf(lastPart, pFrom2);
        if (St2.Substring(pFrom2, pTo2 - pFrom2).Contains(secondPart))
        {
            cleanCombo2 = St2.Substring(pFrom2, pTo2 - pFrom2);
        }
        String St3 = plHTMLP3.Text;
        int pFrom3 = St3.IndexOf(firstPart) + firstPart.Length;
        int pTo3 = St3.IndexOf(lastPart, pFrom3);
        if (St3.Substring(pFrom3, pTo3 - pFrom3).Contains(secondPart))
        {
            cleanCombo3 = St3.Substring(pFrom3, pTo3 - pFrom3);
        }
        String St4 = plHTMLP4.Text;
        int pFrom4 = St4.IndexOf(firstPart) + firstPart.Length;
        int pTo4 = St4.IndexOf(lastPart, pFrom4);
        if (St4.Substring(pFrom4, pTo4 - pFrom4).Contains(secondPart))
        {
            cleanCombo4 = St4.Substring(pFrom4, pTo4 - pFrom4);
        }
        cleanCombo = cleanCombo1 + cleanCombo2 + cleanCombo3 + cleanCombo4;
        return cleanCombo;
    }

And I have three buttons (at the moment) that use this Method.

Here is an example of the code that one of the buttons contain

private void mButton_Click(object sender, EventArgs e)
{
            String firstPart = "<strong>http://m2.";
            String secondPart = "m.com</strong>";
            String lastPart = "</p>";
            GetTextBetween(firstPart, secondPart, lastPart);
            plHTML.Text = FilterHTML(cleanCombo, "m.com");
}

Not sure if this other Method I made affects the code but here it is

    public string FilterHTML(string cleanCombo, string EndOfString)
    {
        cleanCombo = cleanCombo.Replace("<strong>", "");
        cleanCombo = cleanCombo.Replace("<br />", "");
        cleanCombo = cleanCombo.Replace("</strong>", "");
        cleanCombo = cleanCombo.Replace(EndOfString, "");
        return cleanCombo;
    }

Answer 1

I think there is a problem with the scope of the variables cleanCombo1 , cleanCombo2 , cleanCombo3 , cleanCombo4 . These should be declared within the scope of the method GetTextBetween . Your newer method also use these variables and if GetTextBetween does not enter any of the if blocks, it will reuse garbage values inside the cleanComboX variables.