you can look into this link and I hope this may be of great help as its was for me.
If you are using the regex given in the above link in Java code then use the following regex
"^(0[1-9]|1[012])[- /.](0[1-9]|[12][0-9]|3[01])[- /.](19|20)\\d\\d$"
As I noticed in your example you are using js function then this use regex
"^(0[1-9]|1[012])[- /.](0[1-9]|[12][0-9]|3[01])[- /.](19|20)\d\d$"
This will match a date in mm/dd/yyyy format from between 1900-01-01 and 2099-12-31, with a choice of four separators.
and not to forget the
return regex.test(testdate);