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 #region 필드
        /// <summary>
        /// 다음 단어를 찾을 때 Index 위치
        /// </summary>
        int seekIndex = 0;
        #endregion

#region public method
        /// <summary>
        /// 해당 키워드들을 원하는 Color로 지정해줍니다.
        /// </summary>
        /// <param name="words"></param>
        /// <param name="color"></param>
        public void SetColorWords(string []words, System.Drawing.Color color)
        {
            for (int i = 0; i < words.Length; i++)
            {
                string word = words[i];
                while (true)
                {
                    int findIndex = 0;

                    findIndex = NextFind(word);

                    //검색된 것이 없다면
                    if (findIndex == -1)
                        break;

                    int result = this.Find(word, findIndex, findIndex + word.Length, RichTextBoxFinds.MatchCase);
                    if (result == -1)
                        break;

                    this.SelectionColor = color;
                    this.DeselectAll();
                    this.SelectionColor = System.Drawing.Color.Black;
                }
                this.seekIndex = 0;
            }
            this.SelectionLength = 0;
        }
        #endregion
        /// <summary>
        /// seekIndex부터 word에 해당하는 시작되는 Index 반환
        /// </summary>
        /// <param name="word"></param>
        /// <returns></returns>
        private int NextFind(string word)
        {
            int i = 0;
            for (i = seekIndex; i < this.Text.Length; i++)
            {
                int wordIndex = 0;
                while (true)
                {
                    if (this.Text[i] == word[wordIndex])
                    {
                        i++;
                        wordIndex++;
                        if (wordIndex == word.Length)
                        {
                            this.seekIndex = i;
                            return i - word.Length;
                        }
                    }
                    else
                    {
                        break;
                    }
                }
            }
            this.seekIndex = i;
            return -1;
        }

결과 화면

TestRichTextBox.zip

using System;
using System.Collections.Generic;
using System.Text;
using System.Windows.Forms;

namespace WebSolus.Windows.Controls
{
    class KeyWordRichTextBox:RichTextBox
    {
        #region 속성
        /// <summary>
        /// 검색할 KeyWord를 가져오거나 설정합니다.
        /// </summary>
        public List<string> KeyWords {get;set; }
        #endregion

        #region 생성자
        /// <summary>
        /// 생성자
        /// </summary>
        public KeyWordRichTextBox()
        {
            InitializeComponent();
        }
        #endregion

        #region InitializeComponent
        /// <summary>
        /// InitializeComponent
        /// </summary>
        private void InitializeComponent()
        {
            this.SuspendLayout();
            this.ResumeLayout(false);
            this.KeyWords = new List<string>();
            this.KeyUp += new KeyEventHandler(KeyWordRichTextBox_KeyUp);
        }
        #endregion

        #region Api Function
        /// <summary>
        /// Caret의 위치를 가져옵니다.
        /// </summary>
        /// <param name="pt">Point 개체</param>
        /// <returns>true 성공,false 실패</returns>
        [System.Runtime.InteropServices.DllImport("user32.dll")]
        private static extern bool GetCaretPos(out System.Drawing.Point pt);
        #endregion

        #region Event
        /// <summary>
        /// 키를 놓을 때 발생합니다.
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void KeyWordRichTextBox_KeyUp(object sender, KeyEventArgs e)
        {
            for (int i = 0; i < this.KeyWords.Count; i++)
            {
                string word = this.KeyWords[i];
                if (this.SelectionStart >= word.Length)
                {
                    int result = this.Find(word, this.SelectionStart - word.Length, this.SelectionStart, RichTextBoxFinds.MatchCase);
                    if (result != -1)
                    {
                        System.Drawing.Point nearPoint = new System.Drawing.Point();
                        GetCaretPos(out nearPoint);
                        int index = this.GetCharIndexFromPosition(nearPoint);
                        this.SelectionColor = System.Drawing.Color.Blue;
                        if (index + 1 == this.TextLength)
                        {
                            this.SelectionStart = index + 1;
                        }
                        else
                        {
                            this.SelectionStart = index;
                        }
                        this.SelectionColor = System.Drawing.Color.Black;
                        this.DeselectAll();

                    }

                }
            }
        }
        #endregion
    }
}

결과 이미지

프레임워크 버젼 : 닷넷 프레임워크 2.0

TestRichTextBox.zip