JavaScript解法

正则表达式匹配，需要注意的是string.match()在无匹配结果是返回null，反之返回结果数组。

const readline = require('readline');

const getCharacterCountInString = (source, target) => {
  const targetRegExp = new RegExp(target, 'ig');
  const matchs = source.match(targetRegExp);
  return matchs === null ? 0 : matchs.length;
};

const printCharacterCountInString = (source, target) => {
  const count = getCharacterCountInString(source, target);
  console.log(count);
};

const init = () => {
  const rl = readline.createInterface({
    input: process.stdin,
    ouput: process.stdout
  });

  let sourceString = '';
  let targetCharacter = '';

  rl.addListener('line', (line) => {
    if (sourceString === '') {
      sourceString = line;
    } else {
      targetCharacter = line;
      rl.close();
      printCharacterCountInString(sourceString, targetCharacter);
    }
  });
};

init();