Since this may leave an extra space, can we assume that {#ANY_TEX#} will never be at the start of string or at the end? Or can it be?. In other words we can also do something like this in your example to eliminate that.
String word = "This is a test {#ANY_TEXT#} string";
String pattern = @"\{[^\}]*\}\s(\w)";
word = Regex.Replace(word, pattern, "$1");
Console.WriteLine(word);
Console.ReadLine();
John Grove - TFD Group, Senior Software Engineer, EI Division, http://www.tfdg.com