string strRegex = "\\bif\\b|\\belse\\b|\\bfor\\b|\\bwhile\\b";
Regex regKeywords = new Regex(strRegex, RegexOptions.IgnoreCase | RegexOptions.Compiled);
Match regMatch;
m_strLine = "многострочный текст с кодом";
for (regMatch = regKeywords.Match(m_strLine); regMatch.Success; regMatch = regMatch.NextMatch())
            {
                int nStart = regMatch.Index;
                int nLenght = regMatch.Length;
            }

string strCode - "многострочный текст с кодом";
regex regKeywords("\\bif\\b|\\belse\\b|\\bfor\\b|\\bwhile\\b|\\bNDS\\b");
smatch regMatch;			
regex_search(strCode, regMatch, regKeywords);