Hex & Base64 Encode Decode

Hex_Base64.js

// Hex to Base64
function hexToBase64(str) {
    return btoa(String.fromCharCode.apply(null,
      str.replace(/\r|\n/g, "").replace(/([\da-fA-F]{2}) ?/g, "0x$1 ").replace(/ +$/, "").split(" "))
    );
}

// Base64 to Hex
function base64ToHex(str) {
    for (var i = 0, bin = atob(str.replace(/[ \r\n]+$/, "")), hex = []; i < bin.length; ++i) {
        let tmp = bin.charCodeAt(i).toString(16);
        if (tmp.length === 1) tmp = "0" + tmp;
        hex[hex.length] = tmp;
    }
    return hex.join(" ");
}

// Demo
let data = 'E6FF00F0';
let d1 = hexToBase64( data );
console.log(d1);
// output: '5v8A8A=='

let d2 = base64ToHex( d1 );
console.log(d2.toUpperCase());
// output: 'E6 FF 00 F0'

i'm sorry, but a i have to ask.
replace(/([\da-fA-F]{2}) ?/g, "0x$1 "). what is this?