JavaScript RegEx: Format string with dynamic length to block format

Question

The following RegEx formats given string to following output block pattern:

123 456 78 90 (= 3 digits 3 digits 2 digits 2 digits )

RegEx:

Answer 1

anubhava's and Wiktor's solutions are clever, but I don't think I'd use a regular expression to do it; the solutions feel too complex to maintain. (This is a judgement call.) Here's an approach that gets the individual digits and inserts spaces after the third digit and after every subsequent digit whose index position is an odd number:

result = [...str].map((d, i) => d + (i === 2 || (i >= 5 && i % 2 === 1) ? " " : "")).join("");

Live Example:

const tests = [
    ["1234", "123 4"],
    ["1234567", "123 456 7"],
    ["123456789", "123 456 78 9"],
    ["1234567890123", "123 456 78 90 12 3"],
];

function format(str) {
    return [...str].map((d, i) => d + (i === 2 || (i >= 5 && i % 2 === 1) ? " " : "")).join("");
}

for (const [str, expected] of tests) {
    const result = format(str);
    console.log(`|${str}|`, "=>", `|${result}|`, result === expected ? "OK" : "*** ERROR");
}

Here's a version that works in ES5-only environments:

result = str.split("").map(function(d, i) {
    return d + (i === 2 || (i >= 5 && i % 2 === 1) ? " " : "");
}).join("");