alternate-regex.js

function alternate(s) {
    let maxlen = 0;
    const combinations = [];
    [...new Set(s)].forEach((v, i, a) => {
        a.slice(i + 1).forEach(v1 => {
            combinations.push([v, v1]);
        })
    });
    combinations.forEach(comb => {
        let t = [...s].filter(v => comb.includes(v)).join('');
        if (t.match(/(.)\1/) === null) {
            maxlen = Math.max(maxlen, t.length)
        }
    })
    return maxlen;
}


// Explanation:

// A) get a set of unique letters in the string

// B) build an array of all 2 letter combinations from that set

// C) go through those combinations one at a time, building a new string that only consists of those 2 letters

// D) use a regex to see if there are any consecutive matching letters, e.g. aa. If there are then it isn't an alternating string

// E) if it is an alternating string, then update the max length found if it is longer



// (.) matches any character (the dot) and captures it (the brackets). This capture can be used later in the regexp by referring to \1 (backreference capture number one).

// So this regexp matches any character that is immediately followed by the same character, e.g. aa, bb, cc, but not xy

// If the output of that match is null then there are no duplicate characters like that, which therefore means the characters must be alternating.