Question 174690 in Javascript Question posted in
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Javascript – Obtain substrings from matching regex

I have this regex

const regex = new RegExp(/^${[a-z][a-z0-9_]*}/${[a-z][a-z0-9_]*}$/, 'g');

that matches the string "${total_123}/${number_items}". Next, I want to extract the substrings total123 and number_items and set them as

const numerator = total_123 and const denominator = number_items. I’m not exactly sure how to do so.

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3

Answers


  1. 0 
    const regex = new RegExp(/^${([a-z][a-z0-9_]*)}/${([a-z][a-z0-9_]*)}$/, 'g');
const string = "${total_123}/${number_items}";

const matches = string.match(regex);

if (matches) {
  const numerator = matches[1];
  const denominator = matches[2];

  console.log("numerator:", numerator); // "total_123"
  console.log("denominator:", denominator); // "number_items"
}

    The parts enclosed in parentheses in the regex pattern represent groups. These groups can be individually captured using matches array, with the elements at index 1 and 2. This allows you to capture the expressions enclosed in parentheses separately.

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  2. 0

    You need to store the variables in an object, then access its properties as needed:

    const variables = {
  total_123: 42,
  number_items: 0xf00
};

// Destructure the match, take the second and third element but skip the first.
const [, group1, group2] = text.match(regex);

const numerator = variables[group1];
const denominator = variables[group2];

    Also, just declare your regex using the literal syntax:

    const regex = /^${([a-z][a-z0-9_]*)}/${([a-z][a-z0-9_]*)}$/;

    Do note that, since your regex doesn’t have a m flag, ^ means the beginning of the string and $ the end. That being said, we can get at most one match, and the g flag is not necessary (it even prevents us to access the groups if we use .match() and not .matchAll(), in fact).

    Try it:

    console.config({ maximize: true });

const regex = /^${([a-z][a-z0-9_]*)}/${([a-z][a-z0-9_]*)}$/;
const text = '${total_123}/${number_items}';

const variables = {
  total_123: 42,
  number_items: 0xf00
};

const match = text.match(regex);
console.log({match});

// fullMatch is not needed, it's just there for the sake of readability.
const [_fullMatch, group1, group2] = match;
console.log({group1, group2});

const numerator = variables[group1];
const denominator = variables[group2];

console.log(`${numerator}/${denominator}`);
    <script src="https://gh-canon.github.io/stack-snippet-console/console.min.js"></script>
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  3. 0

    First some remarks:

    • You don’t need to call the RegExp constructor. Just use a regex literal.
    • As you can have more than one match with g flag, you need a loop
    • You could use named capture groups, like (?<numerator> )

    Code:

    const regex = /^${(?<numerator>[a-z][a-z0-9_]*)}/${(?<denominator>[a-z][a-z0-9_]*)}$/g;
const s = "${total_123}/${number_items}";

for (const {groups: {numerator, denominator}} of s.matchAll(regex)) {
    console.log(numerator, denominator);
}
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