japaneseRegex.js

// REFERENCE UNICODE TABLES: 
// http://www.rikai.com/library/kanjitables/kanji_codes.unicode.shtml
// http://www.tamasoft.co.jp/en/general-info/unicode.html
//
// TEST EDITOR:
// http://www.gethifi.com/tools/regex
//
// UNICODE RANGE: DESCRIPTION
// 
// 3000-303F: punctuation
// 3040-309F: hiragana
// 30A0-30FF: katakana
// FF00-FFEF: Full-width roman + half-width katakana
// 4E00-9FAF: Common and uncommon kanji
// 
// Non-Japanese punctuation/formatting characters commonly used in Japanese text
// 2605-2606: Stars
// 2190-2195: Arrows
// u203B: Asterisk

var regex = /[\u3000-\u303F]|[\u3040-\u309F]|[\u30A0-\u30FF]|[\uFF00-\uFFEF]|[\u4E00-\u9FAF]|[\u2605-\u2606]|[\u2190-\u2195]|\u203B/g; 
var input = "input string"; 
if(regex.test(input)) {
    console.log("Japanese characters found")
}
else {
    console.log("No Japanese characters");
}

// As a side note, the characters from 0x4e00 to 0x9faf include Chinese-only characters. The following code will give you a list of the standard 6355 Japanese kanji:

for (var i = 0x4e00, acc=[]; i < 0x9faf; i++) acc.push(String.fromCharCode(i));
var sortedChars = acc.sort(Intl.Collator("ja-JP").compare);
var level1Kanji = sortedChars.slice(0, 2965); // JIS X 0208 - Level 1 Kanji (2965 characters)
var level2Kanji = sortedChars.slice(2965, 6355) // JIS X 0208 - Level 2 Kanji (3390 characters)