Describe the procedure of extracting a query string with regular expressions

Last Updated : 24 Apr, 2025

A query string is a part of a URL that follows a question mark (?) and is used to pass data to a web page or application. It is typically composed of key-value pairs that are separated by an ampersand (&). The key represents the parameter and the value represents the data that is being passed through the parameter.

In this article, we will discuss how to extract a query string from a URL using regular expressions.

Approach:

First, we need to define a regular expression pattern that will match the query string of a URL. A regular expression is a sequence of characters that forms a search pattern. It can be used to check if a string contains the specified search pattern.
The regular expression pattern for a query string is:

[?&]([^=]+)(=([^&#]*))?

This pattern matches the beginning of the query string (?), followed by any key-value pairs separated by an ampersand (&). The key is captured in the first capturing group ([^=]+), and the value is captured in the third capturing group (([^&#]*)).

Next, we can use the RegExp object in JavaScript to create a regular expression from the pattern. We can do this by passing the pattern to the RegExp constructor as follows:

const pattern = '[?&]([^=]+)(=([^&#]*))?'; const regex = new RegExp(pattern);

Once we have the regular expression, we can use the test() method to check if the query string of a URL matches the pattern. The test() method returns a boolean value indicating whether the string contains a match or not.

Example 1: The following code is using JavaScript to extract the query string from a given URL by matching it against a regular expression pattern, and logs the query string to the console if it matches.

C++

#include <iostream> #include <regex> #include <string>  using namespace std;  int main() {     // URL to be queried     string url = "https://example.com?key1=value1&key2=value2";      // Regular expression pattern     string pattern = "[?&]([^=]+)(=([^&#]*))?";      // Creating a regex object from the pattern     regex regexObj(pattern);      // Creating a match object to hold the matched substring     smatch match;      // Querying the string using the regex     if (regex_search(url, match, regexObj)) {         string queryString = match.str();         cout << queryString << endl;     }      return 0;  }

Java

import java.util.regex.Matcher; import java.util.regex.Pattern;  public class Main {     public static void main(String[] args)     {         // url to be queried         final String url             = "https:example.com?key1=value1&key2=value2";          // pattern         final String pattern = "[?&]([^=]+)(=([^&#]*))?";          // building regex from the pattern         Pattern regex = Pattern.compile(pattern);          // querying the string using the regex         Matcher matcher = regex.matcher(url);         if (matcher.find()) {             String queryString = matcher.group();             System.out.println(queryString);         }     } }

JavaScript

<script>     // url to be queried     const url = 'https://example.com?key1=value1&key2=value2';      // pattern     const pattern = '[?&]([^=]+)(=([^&#]*))?';      // building regex from the pattern     const regex = new RegExp(pattern);      // querying the string using the regex     if (regex.test(url)) {       const queryString = url.match(regex)[0];       console.log(queryString);        } </script>

using System; using System.Text.RegularExpressions;  class Program {   static void Main(string[] args)   {     // url to be queried     const string url = "https://example.com?key1=value1&key2=value2";      // pattern     const string pattern = @"[?&]([^=]+)(=([^&#]*))?";      // building regex from the pattern     Regex regex = new Regex(pattern);      // querying the string using the regex     if (regex.IsMatch(url))     {       string queryString = regex.Match(url).Value;       Console.WriteLine(queryString);     }   } }

Python

import re  # URL to be queried url = "https://example.com?key1=value1&key2=value2"  # Regular expression pattern pattern = r"[?&]([^=]+)(=([^&#]*))?".encode('unicode-escape').decode()  # Creating a regex object from the pattern regex_obj = re.compile(pattern)  # Querying the string using the regex match = regex_obj.search(url) if match:     query_string = match.group()     print(query_string) # This code is contributed by Prajwal Kandekar

Output

?key1=value1

Example 2: This code is using JavaScript to extract the query parameters from a given URL by matching it against a regular expression pattern, then it splits the query string into individual key-value pairs. It iterates over the key-value pairs and stores them in an object, finally, it logs the query parameters object to the console.

JavaScript

<script>      // Define the URL and the regular expression pattern     const url = 'https://example.com?key1=value1&key2=value2';     const pattern = '[?&]([^=]+)(=([^&#]*))?';      // Create the regular expression object     const regex = new RegExp(pattern);      // Check if the URL matches the pattern     if (regex.test(url)) {        // Extract the query string from the URL        const queryString = url.match(regex)[0];         // Split the query string into individual key-value pairs        const keyValuePairs = queryString.split('&');         // Iterate over the key-value pairs      // store them in an object        const queryParams = {};        keyValuePairs.forEach(pair => {         const [key, value] = pair.split('=');         queryParams[key] = value;       });         // Output the query parameters object       console.log(queryParams);    } </script>

Output

{ '?key1': 'value1' }

Example 3: This code is using JavaScript to extract the query parameters from a given URL by matching it against a regular expression pattern, then it splits the query string into individual key-value pairs, then it iterates over the key-value pairs, and stores them in an object. Finally, it iterates over the query parameters object and outputs each key-value pair in the console.

JavaScript

<script>      // Define the URL and the regular expression pattern     const url = 'https://example.com?key1=value1&key2=value2';     const pattern = '[?&]([^=]+)(=([^&#]*))?';      // Create the regular expression object     const regex = new RegExp(pattern);      // Check if the URL matches the pattern     if (regex.test(url)) {           // Extract the query string from the URL           const queryString = url.match(regex)[0];          // Split the query string into individual key-value pairs        const keyValuePairs = queryString.split('&');       // Iterate over the key-value pairs       // store them in an object      const queryParams = {};      keyValuePairs.forEach(pair => {         const [key, value] = pair.split('=');         queryParams[key] = value;       });        // Iterate over the query parameters object      // output each key-value pair     for (const [key, value] of Object.entries(queryParams)) {         console.log(`${key}: ${value}`);       }   }  </script>

Output

?key1: value1

In conclusion, extracting a query string from a URL using regular expressions is a useful technique for parsing and manipulating data passed through a URL. By defining a regular expression pattern that matches the query string of a URL and using the RegExp object and the test() method, we can easily extract the query string and split it into individual key-value pairs. These key-value pairs can then be stored in an object for easy access, allowing us to easily retrieve and manipulate the data passed through the query string. Regular expressions are a powerful tool for working with strings, and this technique can be useful in a variety of web development scenarios.

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Describe the procedure of extracting a query string with regular expressions

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