[Solved] Develop Regular Expression that every A is followed by a B

by


I know the OP says it has to be a regex, but I agree with @tbodt’s comment above. This will accomplish what the OP is asking for without a regex and is posted here for anyone trying to solve a similar problem and is open to a different type of solution.

function everyAHasMatchingB(str) {
    return Array.from(str)
                .filter(c => c === 'A' || c === 'B')
                .reduce((count, aOrB) => {
                    return aOrB === 'A' ? count + 1 :
                           count <= 0   ? NaN // Mismatched B.  NaN ensures the function returns false.
                                        : count - 1
                }, 0) === 0;
}

solved Develop Regular Expression that every A is followed by a B