打印除字母外不含任何其他字符的字符串，并将空格替换为下划线

function printString(str) {
  let result = '';
  for (let i = 0; i < str.length; i++) {
    if (str[i].match(/[a-zA-Z]/)) {
      result += str[i];
    } else if (str[i] === ' ') {
      result += '_';
    }
  }
  console.log(result);
}

let str = "Hello World!";
printString(str);

Hello_World