How do I pass a variable into regex with Node js?

Question

So basically, I have a regular expression which is

var regex1 = /10661\" class=\"fauxBlockLink-linkRow u-concealed\">([\s\S]*?)<\/a>/;

var result=text.match(regex1);
user_activity = result[1].replace(/\s/g, "")
console.log(user_activity);

What I'm trying to do is this

var number = 1234;
var regex1 = /${number}\" class=\"fauxBlockLink-linkRow u-concealed\">([\s\S]*?)<\/a>/;

but it is not working, and when I tried with RegExp, I kept getting errors.

You can programmatically build your regex string using templates or string addition and pass that string to the `new RegExp(...)` constructor. — jfriend00, May 08 '22 at 23:04
Folks who voted to dup this, please find a better dup. That was a python question, not a Javascript question. Things are not exactly the same. — jfriend00, May 09 '22 at 01:40

score 1 · Answer 1 · answered May 08 '22 at 23:10

1

You can use RegExp to create regexp from a string and use variables in that string.

var number = 1234;
var regex1 = new RegExp(`${number}aa`);

console.log("1234aa".match(regex1));

answered May 08 '22 at 23:10

Adem Çırak

221
2
4

I tried already, this is what my code looks like ``` var text = fs.readFileSync('data.txt','utf8') var number = 1234; var regex1 = new RegExp(``([\s\S]*?)<\/a>``); var result=text.match(regex1); console.log(result); ``` I get 'null' as output – Kemmo Antro May 08 '22 at 23:15
I tried your regex, It looks like the problem was the escapes and _([\s\S]*?)_ part. It requires double escapes. I put a sample on the link https://jsfiddle.net/fhb9jv0k/2/ – Adem Çırak May 09 '22 at 00:01
I'm not sure. I tried your code, its not grouping what is supposed to be between the ''([\s\S]*?)'', its just typing it in full, when I tried to use [] it did not match – Kemmo Antro May 09 '22 at 22:22
https://jsfiddle.net/nucremp9/1/ this is what I mean – Kemmo Antro May 09 '22 at 22:27
https://jsfiddle.net/6ocga3f7/ what I'm trying to extract is '1,193' when I insert the full actual html code on my PC, it doesn't work, on the website it does – Kemmo Antro May 09 '22 at 22:32
All good, I figured it out thanks a lot, you code helped, the issue was in the way i was reading the file, so the content the script was being performed on was wrong all the time – Kemmo Antro May 09 '22 at 22:49

jfriend00 · Accepted Answer · 2022-05-09T02:55:30.370

You can build the regex string with templates and/or string addition and then pass it to the RegExp constructor. One key in doing that is to get the escaping correct as you need an extra level of escaping for backslashes because the interpretation of the string takes one level of backslash, but you need one to survive as it gets to the RegExp contructor. Here's a working example:

function match(number, str) {
    let r = new RegExp(`${number}" class="fauxBlockLink-linkRow u-concealed">([\\s\\S]*?)<\\/a>`);
    return str.match(r);
}


const exampleHTML = '<a href="something" class="aaa1234" class="fauxBlockLink-linkRow u-concealed">Some link text</a>';

console.log(match(1234, exampleHTML));

Note, using regex to match HTML like this becomes very order-sensitive (whereas the HTML itself isn't order-sensitive). And, your regex requires exactly one space between classes which HTML doesn't. If the class names were in a slightly different order or spacing different in the <a> tag, then it would not match. Depending upon what you're really trying to do, there may be better ways to parse and use the HTML that isn't order-sensitive.

score 0 · Answer 3 · answered May 09 '22 at 22:51

I solved it with the method of Adem,

function escapeRegExp(string) {
    return string.replace(/[.*+?^${}()|[\]\\]/g, '\\$&'); // $& means the whole matched string
}


var number = 1234; 

var firstPart = `<a href="/forum/search/member?user_id=${number}" class="fauxBlockLink-linkRow u-concealed">`

var regexpString = escapeRegExp(firstPart) + '([\\s\\S]*?)' + escapeRegExp('</a>');

console.log(regexpString)


var sample = `<a href="/forum/search/member?user_id=1234" class="fauxBlockLink-linkRow u-concealed"> </a>`

var regex1 = new RegExp(regexpString); 

console.log(sample.match(regex1));

in the first place the issue was actually the way I was reading the file, the data I was applying the match on, was undefined.

How do I pass a variable into regex with Node js?

3 Answers3