string str = "part1.abc.part2.abc.part3.abc";
int ix1 = str.LastIndexOf('.');
int ix2 = ix1 > 0 ? str.LastIndexOf('.', ix1 - 1) : -1;
There are always lovers of Regexes (and jQuery), so I'll give a Regex solution (for the jQuery solution you'll have to wait :-) ):
var match = Regex.Match(str, @".[^.]*.", RegexOptions.RightToLeft);
int ix = match.Success ? match.Index : -1;
(note that I'm an hater of Regexes, I'm giving it to you so that you can have enough rope to hang yourself if you so choose).
Be aware that I'm using the RegexOptions.RightToLeft
option so that the Regex starts at the last character.
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