string content = "网站内容";
string pattern = @"<a[\s\S]*?href=(""(?<href>[^""]*)""|’(?<href>[^’]*)’|(?<href>[^>\s]*))[^>]*?>";
        MatchCollection mc = Regex.Matches(content,pattern,RegexOptions.IgnoreCase|RegexOptions.Compiled);
        foreach (Match m in mc)
        {
            string url = m.Groups["href"].Value;
            string replaceUrl = "http://www.baidu.com/" + "?to=" + url;
            content = Regex.Replace(content,url,replaceUrl);
        }

string content = "网站内容";
        string pattern = @"<a[\s\S]*?href=(""(?<href>[^""]*)""|’(?<href>[^’]*)’|(?<href>[^>\s]*))[^>]*?>";
        string repStr = "<a target=\"_blank\" href=\"http://www.baidu.com/?to=${href}\">";
        content = Regex.Replace(content,pattern,repStr,RegexOptions.IgnoreCase|RegexOptions.Compiled);

public static void Main() 
{ 
string s = "1 12 3 5"; 

s = Regex.Replace(s,@"\d+",new 
MatchEvaluator(CorrectString),RegexOptions.Compiled|RegexOptions.IgnoreCase); 

Console.WriteLine(s); 
Console.ReadLine(); 
} 
private static 
string CorrectString(Match match) 
{ 
string matchValue = match.Value; 

if(matchValue.Length == 1) 
matchValue = "0" + matchValue; 
return 
matchValue; 
}

public static void Main() 
{ 
string s = "1 12 3 5"; 
s = 
Regex.Replace(s,@"(\d+)(?#这个是注释)","0$1",RegexOptions.Compiled|RegexOptions.IgnoreCase); 

Console.WriteLine(s); 
Console.ReadLine(); 
}