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How to Extract Numbers from a String in JavaScript

Extracting numeric values from a string is a common and essential task in data parsing. You might need to get a user ID from a log entry, a price from a product description, or a version number from a filename. The most powerful and flexible tool for all of these tasks is a regular expression.

This guide will teach you the modern, standard methods for using regular expressions to extract all numbers, the first number, or the last number from any string.

Goal 1: Extract All Numbers from a String​

If you need to get every number present in a string, the String.prototype.match() method combined with a global regular expression is the perfect tool.

We use a regular expression to find all sequences of digits and then convert them to numbers.

  1. match(/\d+/g): This finds all occurrences of one or more digits (\d+) in the string. The g (global) flag is essential for finding all matches, not just the first.
  2. map(Number): The match() method returns an array of strings. We then use map(Number) to convert each string in that array to a number.

For example, we want to get all the numbers from a string, ignoring the text.

// Problem: How to get [123, 456] from this string?
const str = 'User ID: 123, Order ID: 456';

Recommended Solution:

function getAllNumbers(str) {
  // Find all sequences of one or more digits
  const matches = str.match(/\d+/g);

  // If no matches are found, return an empty array
  if (!matches) {
    return [];
  }

  // Convert the array of string matches to an array of numbers
  return matches.map(Number);
}

// Example Usage:
const str = 'User ID: 123, Order ID: 456';
const numbers = getAllNumbers(str);
console.log(numbers);                       // Output: [123, 456]

const noNumbersStr = 'hello world';
console.log(getAllNumbers(noNumbersStr));   // Output: []

Goal 2: Extract Only the First Number from a String​

To get only the first number that appears in a string, we use a similar match() approach but without the global (g) flag.

The match() method, when used without the global flag, returns null if no match is found, or a "match object" (an array with extra properties) if a match is found. The first number will be at index 0 of this array.

We need to find only the first numeric value in a string.

// Problem: How to get just the number 100 from this string?
const str = 'Product: ABC, Price: 100, Stock: 50';

Recommended Solution:

function getFirstNumber(str) {
  // The regex /\d+/ finds the first sequence of one or more digits.
  const match = str.match(/\d+/);

  // If a match is found, convert it to a number. Otherwise, return null.
  return match ? Number(match[0]) : null;
}

// Example Usage:
const str = 'Product: ABC, Price: 100, Stock: 50';
const firstNumber = getFirstNumber(str);
console.log(firstNumber); // Output: 100

Goal 3: Extract Only the Last Number from a String​

To get only the last number from a string, we can use a regular expression that matches all characters up to the final group of digits. This works by using a greedy pattern (.*) followed by a capturing group for digits (\d+).

The regex .*(\d+) finds the last number in a string.

  • .*: a Greedy match for any character (.), zero or more times (*). This will consume as much of the string as possible.

  • (\d+): a capturing group that matches one or more digits. Because .* is greedy, this captures the last number in the string.

Recommended Solution:

function getLastNumber(str) {
  // The regex captures the last number in the string
  const match = str.match(/.*?(\d+)(?!.*\d)/);
  
  // Return the captured number or null if not found
  return match ? Number(match[1]) : null;
}

// Example Usage:
const str = 'Product: ABC, Price: 100, Stock: 50';
const lastNumber = getLastNumber(str);
console.log(lastNumber); // Output: 50

Conclusion​

Regular expressions provide a powerful and flexible toolkit for extracting numbers from strings.

  • To get all numbers, use str.match(/\d+/g) and then convert the resulting array of strings to numbers.
  • To get the first number, use str.match(/\d+/) and access the result at index 0.
  • To get the last number, use the greedy str.match(/.*(\d+)/) pattern and access the captured group at index 1.

By choosing the right pattern, you can reliably parse any string to find the numeric data you need.

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