parser.js

function insertExplicitConcatOperator(exp) {
    let output = '';

    for (let i = 0; i < exp.length; i++) {
        const token = exp[i];
        output += token;

        if (token === '(' || token === '|') {
            continue;
        }

        if (i < exp.length - 1) {
            const lookahead = exp[i + 1];

            if (lookahead === '*' || lookahead === '?' || lookahead === '+' || lookahead === '|' || lookahead === ')') {
                continue;
            }

            output += '.';
        }
    }

    return output;
};

function peek(stack) {
    return stack.length && stack[stack.length - 1];
}

const operatorPrecedence = {
    '|': 0,
    '.': 1,
    '?': 2,
    '*': 2,
    '+': 2
};

function toPostfix(exp) {
    let output = '';
    const operatorStack = [];

    for (const token of exp) {
        //if token is an operator
        if (token === '.' || token === '|' || token === '*' || token === '?' || token === '+') {
            while (operatorStack.length && peek(operatorStack) !== '('
                && operatorPrecedence[peek(operatorStack)] >= operatorPrecedence[token]) {
                output += operatorStack.pop();
            }

            operatorStack.push(token);
        } else if (token === '(' || token === ')') {
            if (token === '(') {
                operatorStack.push(token);
            } else {
                while (peek(operatorStack) !== '(') {
                    output += operatorStack.pop();
                }
                operatorStack.pop();
            }
        } else {
            output += token;
        }
    }

    while (operatorStack.length) {
        output += operatorStack.pop();
    }

    return output;
};
module.exports = {
    toPostfix,
    insertExplicitConcatOperator
}