how to find a specif word in a richtext box libe by line click

Here's a sketch of some code which, I hope, will get you started.

Put a RichTextBox, a Button, and a TextBox on a WinForm: name them: 'richTextBox1, 'button1, and 'textBox1:

private string searchTerm;

private string theLine;

private int maxLines;
private int targetLine = 0;
private int matchAt;
private int searchTermLength;
private int lineLength;
private int lineOffset = 0;

private void button1_Click(object sender, EventArgs e)
{
    maxLines = richTextBox1.Lines.Length;

    searchTerm = textBox1.Text;

    searchTermLength = searchTerm.Length;

    theLine = richTextBox1.Lines[targetLine];

    lineLength = theLine.Length;

    if(theLine.Contains(searchTerm))
    {
         matchAt = theLine.IndexOf(searchTerm);

         richTextBox1.Select(lineOffset + matchAt, searchTermLength);

         richTextBox1.SelectionBackColor = Color.Yellow;
         
         // at this point you really have all the information you need to 
         // to keep scanning the current line for more search term matches !
         // personally, I'd use a while loop starting here ...
    }
 
    // note we don't do anything here if no match is found in the current line

    targetLine ++;

    // consider why we must add #1 here
    lineOffset += lineLength + 1;

    if (targetLine == maxLines)
    {
        targetLine = 0;
        lineOffset = 0;
    }
}

This code example will find, line by line, the first example of the search term in the RichTextBox, and highlight it yellow.

What it does not do is:

1. give any indication that it just checked a line which did not have the search term: if you want it to "skip over" lines that have no search term match, and automatically advance to the next line: that's left for you to do.

2. find multiple matches of the search term: that's left to you to figure out, but once you have "isolated" the current line, and have the information you can see calculated in the code: I believe that will not be difficult.

3. deal with the case where the RichTextBox content is changed, and you need to reset all the parameters for searching. And un-highlight any currently highlighted matches ?

4. deal with the case where the search term has changed, and you need to reset all the parameters for searching. And un-highlight any currently highlighted matches ?

5. any of the type of error-checking and validation a truly robust solution would do.

A really "robust" solution may need to keep track of all currently highlighted match terms, so it's easy to un-highlight them ? I might consider building a List<int> of each highlight match, where the integer value was the absolute value of the offset of the start of the match in the RichTextBox.Text: given the assumption that the search-term length is a constant, then all you need to do is execute a foreach loop through the generic List<int>, and select the high-lighted word, and reset the color.

Or, much more simply, just select all the Text in the RichTextBox and change its SelectionBackColor to some default value ?