@Blog

        function moji(o) {
            let m = o.nextElementSibling;
            //ads
            let h = ((o) => {
                let l = o.value.match(/(https?:\/\/[a-z|A-Z|0-9|\-|_|%|\.|\/]{0,})/giu);
                let ml = l ? ((l) => {
                    return sum = l.reduce((s, e) => {
                        return s + e.length;
                    }, 0);
                })(l) : 0;
                return l ? {
                    len: (l.length * 23),
                    mlen: ml
                } : {
                    len: 0,
                    mlen: ml
                };
            })(o);
            //zenkaku            
            let k = ((o) => {
                let l = o.value.match(/[^\x20-\x7e]/giu);
                let ml = l ? ((l) => {
                    return sum = l.reduce((s, e) => {
                        return s + e.length;
                    }, 0);
                })(l) : 0;
                return l ? {
                    len: (l.length * 2),
                    mlen: l.length
                } : {
                    len: 0,
                    mlen: ml
                };
            })(o);
            m.innerHTML = "【 " + (o.value.length + h.len - h.mlen + k.len - k.mlen) + "文字{半角/280} 】";
            if ((o.value.length + h.len - h.mlen + k.len - k.mlen) >= 280) {
                m.innerHTML = "<span class='text-danger'>【 " + (o.value.length + h.len - h.mlen + k.len - k.mlen) + "文字{半角/280} 】</span>";
            }
        }

                    <label for="my-textarea" class="h3">文字を入力してください。</label>
                    <textarea id="my-textarea" class="form-control" oninput="moji(this);" name="" rows="3"></textarea>
                    <span class="h3"></span>