问替换两个选项之间的字符的正则表达式

EN

const func = str => str.replace(/:\d\d\s*(\w)\.?\s*(\w)\.?/, (a, b, c) => {
  return ` ${b}${c}`.toUpperCase();
});

console.log(func('12:12:34 a. m.'));
console.log(func('12:34:56 a.m.'));
console.log(func('12:12:34 pm'));
console.log(func('12:34:56pm'));
console.log(func('12:34:56p.   m'));

function replaceTime(str) {
  return str.replace(/(\d{2}:\d{2}):\d{2}/, "$1")
            .replace(/\s*a\.*\s*m\.*/, " AM");
}
console.log(replaceTime("12:00:00 a. m."));
console.log(replaceTime("12:00:23 a.m"));

str.replace(/^(.*):\d+/, '$1')
    .replace(/([a-z])[. ]*/g, ($0, $1) => $1.toUpperCase())

console.log("12:00:00 a. m.".replace(/^(.*):\d+/, '$1')
        .replace(/([a-z])[. ]*/g, function($0, $1) { return $1.toUpperCase(); }))