let input = "1d6 + 2d8 + 5";

//let rollEXP = /[^ ,.+]+/gi;
const rollEXP = /(?:[0-9]*d[0-9]+)|(?:[0-9]+)/gi;

let segments = input.match(rollEXP);

let max = new Array(segments.length);
let min = new Array(segments.length);
let avg = new Array(segments.length);

const segmentEXP = /([0-9]*)d([0-9]+)/i;

function parz(i)
{
    let segment = segments[i];

    let parts = segment.match(segmentEXP);
    if (parts != null)
    {
        let numDice;
        let numFace = parseInt(parts[2]);

        if (parts[1] === "") numDice = 1;
        else numDice = parseInt(parts[1]);

        max[i] = numFace*numDice;
        min[i] = numDice;
        avg[i] = (numDice*(numFace+1))/2;
    } else
    {
        min[i] = max[i] = avg[i] = parseInt(segment);
    }
}

for (let i = 0; i < segments.length; ++i) parz(i);

function sum(array) {
    let t = 0;
    array.forEach((val) => {
        t += val;
    });

    return t;
}

let MAX = sum(max);
let MIN = sum(min);
let AVG = sum(avg);

/*
For input = "1d6 + 2d8 + 5"
MAX = 27
MIN = 8
AVG = 17.5
*/