奇怪的JS正则之 /[A-z]/.test("\\"); // true

本文是在一个国外介绍JS的网站上转载过来的，作者很逗，先是举例JS让人XX的例子，再动手实践发现JS隐藏的黑知识。为什么 /[A-z]/.test("\\"); 是 true ，你看懂了么？

原文。

When I use regular expressions and I want to validate a range of letters, I can do it using a-z or A-Z. Even when I use A-z it works fine too. The problem comes doing some test:

/[A-Z]/.test("A"); // true
/[A-Z]/.test("b"); // false
/[A-Z]/.test("Z"); // true
/[A-Z]/.test("z"); // false
/[a-z]/.test("a"); // true
/[a-z]/.test("A"); // false
/[a-z]/.test("z"); // true
/[a-z]/.test("Z"); // false

The weird thing comes when I do this test:

/[A-z]/.test("A"); // true
/[A-z]/.test("a"); // true
/[A-z]/.test("Z"); // true
/[A-z]/.test("z"); // true
/[A-z]/.test("m"); // true
/[A-z]/.test("D"); // true
/[A-z]/.test("\\"); // true WTF?

It's supposed to accept only letters from A to Z and a to z. Can someone explain this?

— @byoigres

I had a look into this with the following code:

var re = /[A-z]/g,s=(function(){
var f = String.fromCharCode;
for(var i=0;i<6000;i++) f=f.bind(0, i);
return f();
})(),q,z=[];while((q=re.exec(s)) != null) z.push(q[0]);z

It returns

["A", "B", "C", "D", "E", "F", "G", "H", "I", "J", "K", "L", "M", "N", "O",
"P", "Q", "R", "S", "T", "U", "V", "W", "X", "Y", "Z", "[", "\", "]", "^",
"_", "`", "a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l", "m",
"n", "o", "p", "q", "r", "s", "t", "u", "v", "w", "x", "y", "z"]

It is likely, I think that A-z literally means 'any character between 'A' and 'z' in unicode code-point order, or at least charCode order. This allows (I think non-standard) statements like /[ -y]/g:

var re = /[ -y]/g,s=(function(){
var f = String.fromCharCode;
for(var i=0;i<6000;i++) f=f.bind(0, i);
return f();
})(),q,z=[];while((q=re.exec(s)) != null) z.push(q[0]);z

Which returns

[" ", "!", """, "#", "$", "%", "&", "'", "(", ")", "*", "+", ",", "-", ".",
"/", "0", "1", "2", "3", "4", "5", "6", "7", "8", "9", ":", ";", "<", "=",
">", "?", "@", "A", "B", "C", "D", "E", "F", "G", "H", "I", "J", "K", "L",
"M", "N", "O", "P", "Q", "R", "S", "T", "U", "V", "W", "X", "Y", "Z", "[",
"\", "]", "^", "_", "`", "a", "b", "c", "d", "e", "f", "g", "h", "i", "j",
"k", "l", "m", "n", "o", "p", "q", "r", "s", "t", "u", "v", "w", "x", "y"]`

This probably has some potential security implications because if you're using [A-z] to sanitise something, you'll accept []^_`

A very interesting find!

— zemnmez