Our phone extension has been changed from 3 digits to 4 digits. All 3 digits extension need to add number 5

For instance Ext. 234 should be Ext. 5234

I am not sure how to use the following RegEx in jQuery to replace extension

(( |x|ext|ext.|extension|Ext|Ext.|Extension|#){1}[ ]?([0-9]){4}){0,1}$