Reverse Only Letter in a String

reverse-only-letters.js

/**
 * @param {string} S
 * @return {string}
 * "ab-cd" should become "dc-ba"
 */
var reverseOnlyLetters = function(S) {
    let stack = [];
    let resultArr = [];
    let letters = /^[A-Za-z]+$/;
    
    for(let i = 0;i<S.length;i++){
        if(S[i].match(letters)){
            stack.push(S[i])
        }
    }
    
    for(let i = 0;i<S.length;i++) {
        if(S[i].match(letters)){
            resultArr.push(stack.pop())
        }else{
            resultArr.push(S[i])
        }
    }
    return resultArr.join('');
};