@Hunter is right.
In addition i found complete implementation in C#: http://weblogs.asp.net/farazshahkhan/archive/2008/08/09/regex-to-find-url-within-text-and-make-them-as-link.aspx.
In case original link goes down
VB.Net implementation
Protected Function MakeLink(ByVal txt As String) As String
Dim regx As New Regex("http://([\w+?\.\w+])+([a-zA-Z0-9\~\!\@\#\$\%\^\&\*\(\)_\-\=\+\\\/\?\.\:\;\'\,]*)?", RegexOptions.IgnoreCase)
Dim mactches As MatchCollection = regx.Matches(txt)
For Each match As Match In mactches
txt = txt.Replace(match.Value, "<a href='" & match.Value & "'>" & match.Value & "</a>")
Next
Return txt
End Function
C#.Net implementation
protected string MakeLink(string txt)
{
Regex regx = new Regex("http://([\\w+?\\.\\w+])+([a-zA-Z0-9\\~\\!\\@\\#\\$\\%\\^\\&\\*\\(\\)_\\-\\=\\+\\\\\\/\\?\\.\\:\\;\\'\\,]*)?", RegexOptions.IgnoreCase);
MatchCollection mactches = regx.Matches(txt);
foreach (Match match in mactches) {
txt = txt.Replace(match.Value, "<a href='" + match.Value + "'>" + match.Value + "</a>");
}
return txt;
}