STA(.*?)M2
is enough, but you need to enable
Singleline[
^].
var s = @"STA;12345;325-5899
ML;2222;5555;GHAA
-;5881;2K5K1KKL
M2;KKN;11232";
var re = new Regex("STA(.*?)M2", RegexOptions.Singleline);
Console.WriteLine(re.Matches(s)[0].Groups[1].Value);
Yields:
;12345;325-5899
ML;2222;5555;GHAA
-;5881;2K5K1KKL