在javascript中解析字符串中的Int

const str = "42 is the answer";
const num = parseInt(str, 10);
console.log(num); // 输出：42

const str = "42 is the answer";
const regex = /\d+/;
const matches = str.match(regex);
const num = matches ? parseInt(matches[0], 10) : null;
console.log(num); // 输出：42

const str = "42 is the answer";
const num = eval(str.replace(/[^\d]/g, ''));
console.log(num); // 输出：42