Highlight keywords by regular expression in C#

public string Highlight(string piInput, string piKeyword) 
{ 
// Setup the regular expression and add the Or operator.
Regex oRegex = new Regex(piKeyword.Replace(" ", "|").Trim(), RegexOptions.IgnoreCase)

// Highlight keywords by calling the delegate each time a keyword is found.
return oRegex.Replace(piInput, new MatchEvaluator(ReplaceKeyWords)); 
}

public string ReplaceKeyWords(Match piMatch) 
{
 return "" + piMatch.Value + ""; 
}