<a href="http://as.xxx.com/" onclick="co('as')">鞍山</a><a href="http://ay.xxx.com/" onclick="co('ay')">安阳
...
...
</a>

        string s=@"<a href=""http://as.xxx.com/"" onclick=""co('as')"">鞍山</a><a href=""http://ay.xxx.com/"" onclick=""co('ay')"">安阳
...
...
</a>";
        MatchCollection matches = Regex.Matches(s, @"(?is)(?<=onclick=""co\(')[^']+(?='\))");
        foreach (Match match in matches)
            Response.Write(match.Value + "<br/>");

------解决方案--------------------
(?<=co\(')[^']+(?='\))