var regex = new RegExp('xyz', 'i');
// 等价于
var regex = /xyz/i;

var regex = new RegExp(/xyz/i);
// 等价于
var regex = /xyz/i;

// Uncaught TypeError: Cannot supply flags when constructing one RegExp from another
var regex = new RegExp(/xyz/, 'i');

console.log(new RegExp(/abc/ig, 'i').flags);//i

//ES5的source属性返回正则表达式的正文
/abc/ig.source//"abc"
// ES6的flags属性返回正则表达式的修饰符
/abc/ig.flags//'gi' 

String.prototype.match 调用 RegExp.prototype[Symbol.match]
String.prototype.replace 调用 RegExp.prototype[Symbol.replace]
String.prototype.search 调用 RegExp.prototype[Symbol.search]
String.prototype.split 调用 RegExp.prototype[Symbol.split]

/^\uD83D/u.test('\uD83D\uDC2A') // false
/^\uD83D/.test('\uD83D\uDC2A') // true

var text = "𠮷" ;
console.log(text.length); // 2
console.log(/^.$/.test(text));//false
console.log(/^.$/u.test(text)); //true

/\u{61}/.test('a') // false
/\u{61}/u.test('a') // true
/\u{20BB7}/u.test('𠮷') // true

/a{2}/.test('aa') // true
/a{2}/u.test('aa') // true
/𠮷{2}/.test('𠮷𠮷') // false
/𠮷{2}/u.test('𠮷𠮷') // true

/^\S$/.test('𠮷') // false
/^\S$/u.test('𠮷') // true

function codePointLength(text) {
 var result = text.match(/[\s\S]/gu);
 return result ? result.length : 0;
}
var s = '𠮷𠮷';
console.log(s.length); // 4
console.log(codePointLength(s)); // 2

function hasRegExpU() {
  try {
    var pattern = new RegExp(".", "u");
    return true;
  } catch (ex) {
    return false;
  }
}

var s = 'aaa_aa_a';
var r1 = /a+/g;
var r2 = /a+/y;
console.log(r1.exec(s)); // ["aaa"]
console.log(r2.exec(s)); // ["aaa"]
console.log(r1.exec(s)); // ["aa"]
console.log(r2.exec(s)); // null

var s = 'aaa_aa_a';
var r = /a+_/y;
console.log(r.exec(s)); // ["aaa_"]
console.log(r.exec(s)); // ["aa_"]

'a1a2a3'.match(/a\d/y) // ["a1"]
'a1a2a3'.match(/a\d/gy) // ["a1", "a2", "a3"]

var r = /hello\d/y;
r.sticky // true

const TOKEN_Y = /\s*(\+|[0-9]+)\s*/y;
const TOKEN_G = /\s*(\+|[0-9]+)\s*/g;
tokenize(TOKEN_Y, '3 + 4')// [ '3', '+', '4' ]
tokenize(TOKEN_G, '3 + 4')// [ '3', '+', '4' ]
function tokenize(TOKEN_REGEX, str) {
 let result = [];
 let match;
 while (match = TOKEN_REGEX.exec(str)) {
  result.push(match[1]);
 }
 return result;
}

tokenize(TOKEN_Y, '3x + 4')
// [ '3' ]
tokenize(TOKEN_G, '3x + 4')
// [ '3', '+', '4' ]

function hasRegExpY() {
  try {
    var pattern = new RegExp(".", "y");
    return true;
  } catch (ex) {
    return false;
  }
}