how to find a particular word from rich text box each line by line click

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C#

WinForms

Hi i have a rich text box if there is 10 sentences in rich text box arrange line by line and i want to find a specific word write in another text box.when i click on button then i want to highlight all search word of first line from rich text box and then when i click on button second time i want to highlight all search words from second line of rich text box.

i am using this code it works .it highlights search word on every click but i want when i click on button it search one line if there are 2 specific words in line one so highlight 2 words.

int textEnd = txtshowfile.TextLength;
                int index = 1;
                int prevIndex = Convert.ToInt16(txtshowfile.Tag);
                int lastIndex = txtshowfile.Text.LastIndexOf(txtsearchgerman.Text);
                txtshowfile.SelectionBackColor = Color.Red;
                prevIndex = txtshowfile.Text.IndexOf(txtsearchgerman.Text, prevIndex + 1);
                txtshowfile.Tag = prevIndex;
                index = prevIndex;
                if (prevIndex < 0)
                {
                    MessageBox.Show("all german words are compare");                    
                }
                else
                {
                    txtshowfile.Find(txtsearchgerman.Text, index, textEnd, RichTextBoxFinds.None);
                    txtshowfile.SelectionBackColor = Color.Yellow;
                    index = txtshowfile.Text.IndexOf(txtsearchgerman.Text, index) + 1;
                    flag1++;
                }
>

i am new in coding.


Thanks in Advance.