Javascript Create Sentence Case Converter with Exceptions

Question

I currently have a function that converts a string to sentence case.

 function toSentenceCase(str){
 convertedTextValueDesc = str.replace(/(^\w{1}|\.\s*\w{1})/gi);

}

This piece of code using RegEx doesn't convert ALL CAPITAL words to sentence case or allow for exceptions. Any help will be much appreciated and Thanks in advance.