正则表达式，javascript背后的负面看法

My problem seems trivial, yet I can't quite seem to grasp what I am doing wrong. I am trying to detect the end of sentences by using a regular expression to find periods. The thing is, I only want say, periods that come after a word with more than 2 letters in it, that way I can avoid a false positive of "St. Nicholas." My problem seems trivial, yet I can't quite see