public string GetTextBetween(string firstPart, string secondPart, string lastPart)
{
    string cleanCombo1 = string.Empty;
    string cleanCombo2 = string.Empty;
    string cleanCombo3 = string.Empty;
    string cleanCombo4 = string.Empty;

    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);
    }

    return cleanCombo1 + cleanCombo2 + cleanCombo3 + cleanCombo4;
}

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

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