Regular Expression, negative look behind javascript

Question

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 current expression is:

/\w{3,}\./g

However, this seems to select the whole word, rather than just the period. What am I doing wrong?

EDIT:
I am expecting

"St. Kitts is really cool. Like seriously, sweet."

To return the period after both "cool" and "sweet", but not after "St."

EDIT 2:
This is in Javascript, so a normal lookbehind of (?<=text) will not work

Answer 1

You can use groups:

var myRegexp = \w{3,}(\.);
var match = myRegexp.exec(your string);
alert(match[1]);  // Here you should have captured the period

And if there is more than one period:

match = myRegexp.exec(myString);
while (match != null) {
    // matched period: match[1]        
    match = myRegexp.exec(myString);
}