source: Common/FormFind.cs

using System;
using System.Drawing;
using System.Windows.Forms;
using Microsoft.Win32;

namespace Common
{
    public partial class FormFind : Form
    {
        public RichTextBoxEx richTextBox;
        public string regSubKey;

        public FormFind()
        {
            InitializeComponent();
            regSubKey = "Find";
        }

        public int FindText(string text, int start, int selectionLength, bool reverse = false)
        {
            int result = -1;
            int end = richTextBox.Text.Length;

            richTextBox.UndoUnknownActions();

            if ((start >= 0) && (end >= start))
            {
                {
                    RichTextBoxFinds options = RichTextBoxFinds.None;
                    if (checkBoxMatchCase.Checked) options |= RichTextBoxFinds.MatchCase;
                    if (checkBoxMatchWholeWordOnly.Checked) options |= RichTextBoxFinds.WholeWord;
                    if (reverse)
                    {
                        end = start;
                        start = 0;
                        options |= RichTextBoxFinds.Reverse;
                    } else
                    {
                        if (selectionLength > 0) start = start + selectionLength;
                    }
                    result = richTextBox.Find(text, start, end, options);
                }
            }
            return result;
        }

        private void textBoxWhat_TextChanged(object sender, EventArgs e)
        {
            UpdateInterface();
        }

        private void SearchText(bool reverse = false)
        {
            comboBoxWhat.UpdateHistory();

            while (true)
            {
                int newPos = FindText(comboBoxWhat.Text.Trim(), richTextBox.SelectionStart, richTextBox.SelectionLength, reverse);
                if (newPos >= 0)
                {
                    richTextBox.SelectionColor = Color.Red;
                    richTextBox.Select(newPos, comboBoxWhat.Text.Length);
                    richTextBox.ScrollToCaret();
                }
                else
                {
                    if (MessageBox.Show("No other occurence found. Continue from start?", "Text search", MessageBoxButtons.YesNo) == DialogResult.Yes)
                    {
                        if (reverse) richTextBox.SelectionStart = richTextBox.Text.Length;
                            else richTextBox.SelectionStart = 0;
                        continue;
                    }
                }
                break;
            }
        }

        private void buttonFindNext_Click(object sender, EventArgs e)
        {
            SearchText();
        }

        private void buttonFindPrevious_Click(object sender, EventArgs e)
        {
            SearchText(true);
        }

        private void FormFind_Load(object sender, EventArgs e)
        {
            Theme.UseTheme(this);
            DpiScaling.Apply(this);
            Icon = Icon.ExtractAssociatedIcon(Application.ExecutablePath);
            new FormDimensions().Load(this, Owner);
            comboBoxWhat.Focus();
            comboBoxWhat.regSubKey = regSubKey + "\\History";
            LoadFromRegistry();
            UpdateInterface();
        }

        private void buttonCancel_Click(object sender, EventArgs e)
        {
            Close();
        }

        private void FormFind_FormClosing(object sender, FormClosingEventArgs e)
        {
            SaveToRegistry();
            richTextBox.FormFind = null;
            new FormDimensions().Save(this, Owner);
        }

        private void textBoxWhat_KeyPress(object sender, KeyPressEventArgs e)
        {
            if (e.KeyChar == 13)
            {
                buttonFindNext.PerformClick();
            } else
            if (e.KeyChar == 27)
            {
                Close();
            }
        }

        private void UpdateInterface()
        {
            buttonFindNext.Enabled = comboBoxWhat.Text.Length > 0;
            buttonFindPrevious.Enabled = comboBoxWhat.Text.Length > 0;
        }

        public void LoadFromRegistry()
        {
            RegistryKey regKey = Application.UserAppDataRegistry.OpenSubKey(regSubKey);
            if (regKey == null) regKey = Application.UserAppDataRegistry.CreateSubKey(regSubKey);

            checkBoxMatchWholeWordOnly.Checked = regKey.GetValueBool("MatchWholeWordOnly", false);
            checkBoxMatchCase.Checked = regKey.GetValueBool("MatchCase", false);

            comboBoxWhat.LoadFromRegistry();
        }

        public void SaveToRegistry()
        {
            RegistryKey regKey = Application.UserAppDataRegistry.OpenSubKey(regSubKey, true);
            if (regKey == null) regKey = Application.UserAppDataRegistry.CreateSubKey(regSubKey);

            
            regKey.SetValue("MatchWholeWordOnly", checkBoxMatchWholeWordOnly.Checked);
            regKey.SetValue("MatchCase", checkBoxMatchCase.Checked);

            comboBoxWhat.SaveToRegistry();
        }

        private void comboBoxWhat_SelectedIndexChanged(object sender, EventArgs e)
        {

        }

        private void label1_Click(object sender, EventArgs e)
        {

        }

        private void checkBoxMatchWholeWordOnly_CheckedChanged(object sender, EventArgs e)
        {

        }

        private void checkBoxMatchCase_CheckedChanged(object sender, EventArgs e)
        {

        }
    }
}