You could remove any other character in the string and check the length:

str.replace(/[^aA]/g, "").length
Here it is counted how many as are in str. The RegExp is described below:

[ // Start Character Group
^ // Not operator in character group
a // The character "a"
] // End character group

//The g (global) in the regular expression tells the program to search 
//the whole string rather than just finding the first occurrence.