var counter = function(str,seq){ return str.split(seq).length - 1; } var str = "Qiita"; var str2 = "Qiiiiiiiiiiiiiiiiiiita"; var str3 = "a1b2c3d4e5f" console.log(str+":"+counter(str,"i")); console.log(str2+":"+counter(str2,"i")); //正規表現も使える console.log(str3+":"+counter(str3,/\d/)); console.log(str3+":"+counter(str3,/[a-z]/));