if yout HTML is perfectly formatted, you can match it using regular expressions if you want! No problem. But @rexcfnghk advice is correct in the pratical world...
If someone wants to use Regular Expression... try this example!
You could explore the collection
better to check if the content is there, of course... just explore the MatchCollection class.
string htmlNode = "<img src=\"%24FILEBASE%moga.jpg\" alt=\"moga.jpg\" width=\"882\" height=\"324\">";
MatchCollection collection = Regex.Matches(htmlNode, "src=\\\"%24FILEBASE%(.*?)\\\"", RegexOptions.IgnoreCase);
string result = collection[0].Groups[1].Value;