RegExp help for converting hyperlinks

I am trying to come up with a regexp and have tried many combinations and searching to find a solution to convert non-hyperlinked addresses to hyperlinks.

ie

http://twitpic.com/abcdef http://www.smh.com.au askjhsd www.hotmail.com ks sd 
<a href="http://www.aaaaaaaa.com">aaaaaaaa</a>

I want the http://twitpic.com/abcdef, http://www.smh.com.au and www.hotmail.com to be picked up but not the http://www.aaaaaaaa.com as it is wrapped around an <a> tag already.

I am currently using this regexp in C#

return Regex.Replace(input, @"(\b((http|https)://|www\.)[^ ]+\b)", 
   @" <a href=""$0"" target=""_blank"">$0</a>", RegexOptions.IgnoreCase);

I have no idea how to make it exclude stuff already wrapped in <a> or <img>

Help :)

EDIT

For those reading this later, this is the final solution I came up with

/// <summary>
/// Adds to the input string a target=_blank in the hyperlinks
/// </summary>
public static string ConvertURLsToHyperlinks(string input)
{
    if (!string.IsNullOrEmpty(input))
    {
        var reg = new Regex(@"(?<!<\s*(?:a|img)\b[^<]*)(\b((http|https)://|www\.)[^ ]+\b)");
        return reg.Replace(input, new MatchEvaluator(ConvertUrlsMatchDelegate));

    }
    return input;
}

public static string ConvertUrlsMatchDelegate(Match m)
{
    // add in additional http:// in front of the www. for the hyperlinks
    var additional = "";
    if (m.Value.StartsWith("www."))
    {
        additional = "http://";
    }
    return "&l开发者_如何转开发t;a href=\"" + additional + m.Value + "\" target=\"_blank\">" + m.Value + "</a>";
}

You could use

@"(?<!<\s*(?:a|img)\b[^<]*)(\b((http|https)://|www\.)[^ ]+\b)"

as your regex. The negative lookbehind assertion.

The lookbehind assertion explained:

(?<!       # Assert that it's impossible to match before the current position:...
 <         # a <
 \s*       # optional whitespace
 (?:a|img) # a or img
 \b        # as an entire word
 [^<]*     # followed by any number of characters except <
)          # end of lookbehind