C#

int posTxt = 0; 
        private void button2_Click(object sender, EventArgs e)
        {
            string searchText = textBox2.Text.ToLower();
            //string alltxt = richTextBox1.Text.ToLower();
            string alltxt = richTextBox1.Text.ToLower();
            RichTextBox rtb = new RichTextBox(); rtb = richTextBox1;

            if (alltxt.Contains(searchText))
            {
                //richTextBox1.Text = alltxt;
                rtb.Text = alltxt;
                
                while (posTxt != -1)
                {
                    posTxt = rtb.Find(searchText, posTxt, RichTextBoxFinds.MatchCase);
                    if (rtb.SelectedText == searchText)
                    {
                        richTextBox1.SelectionLength = searchText.Length;
                        richTextBox1.SelectionColor = Color.Red;
                    }
                    posTxt = rtb.Find(searchText, posTxt + 1, RichTextBoxFinds.MatchCase);
                }
            }
            posTxt = 0;
        }

_q12_ wrote:

The text in richTextBox1 is changing to lower and I dont see from where.

_q12_ wrote:

string alltxt = richTextBox1.Text.ToLower();

_q12_ wrote:

rtb.Text = alltxt;

RichTextBox rtb = new RichTextBox(); rtb = richTextBox1;

_q12_ wrote:

rtb = richTextBox1;

RichTextBox rtb = new RichTextBox(); rtb.Text = richTextBox1.Text;

RichTextBox rtb = new RichTextBox(); rtb = richTextBox1;

richTextBox1.Text = alltxt;

int posTxt = 0; string tempText = "";
        private void button2_Click(object sender, EventArgs e)
        {
            richTextBox1.Text = tempText;//somewhere in the page load : tempText = richTextBox1.Text;

            string searchText = textBox2.Text.ToLower();
            string alltxt = richTextBox1.Text.ToLower();

            if (alltxt.Contains(searchText))
            {
                //problems at start
                if (posTxt <= 0)
                {
                    richTextBox1.Text = richTextBox1.Text.Insert(0, " "); 
                    posTxt = 1;
                }
                //colorization at found text 
                while (posTxt > 0)
                {
                    posTxt = richTextBox1.Find(searchText, posTxt, RichTextBoxFinds.None);
                    if (richTextBox1.SelectedText.ToLower() == searchText)
                    {
                        richTextBox1.SelectionLength = searchText.Length;
                        richTextBox1.SelectionColor = Color.Red;
                    }
                    posTxt = richTextBox1.Find(searchText, posTxt + 1, RichTextBoxFinds.None);
                }
            }
            posTxt = 0;
        }

^\d+$

^[0-9]

if(s.Length == 0) return neither;
else if(s[0] == '-') return negative;
else if(s[0] >= '0' && s[0] <= '9') return positive;
else return non-numeric;