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# # 771 Jewels and Stones

如何帶變數進 RegExp

[LeetCode](https://leetcode.com/problems/jewels-and-stones/)

```
You're given strings J representing the types of stones that are jewels, 
and S representing the stones you have.  
Each character in S is a type of stone you have. 
You want to know how many of the stones you have are also jewels.

The letters in J are guaranteed distinct, and all characters in J and S are letters. Letters are case sensitive, 
so "a" is considered a different type of stone from "A".

input: 給一個 String J 代表鑽石， S 代表石頭 Case sensitive
output: 找出石頭裡面有幾顆鑽石 
```

```
Example 1:

Input: J = "aA", S = "aAAbbbb"
Output: 3

Example 2:

Input: J = "z", S = "ZZ"
Output: 0
Note:

S and J will consist of letters and have length at most 50.
The characters in J are distinct.

/**
 * @param {string} J
 * @param {string} S
 * @return {number}
 */
var numJewelsInStones = function(J, S) {}
```

### 怎麼解

我會用 regex 

```
var numJewelsInStones = function(J, S) {
    let variable = `[${J}]`
    let re = new RegExp(variable, "g")
    return S.match(re) ? S.match(re).length : 0;
};

console.log(numJewelsInStones("z", "ZZ"))
```

### 學到什麼

平常用 regex 都是

```
let re = /[aA]/g
"aAAbbbb".match(re).length
```

沒用過帶變數進去的，原來變數進去需要這樣用

```
let variable = `[${J}]`
let re = new RegExp(variable, "g")
"aAAbbbb".match(re).length

```