URL Parser

Understanding URL Structure

A uniform resource locator (URL) is a structured string that tells a browser or other client where to find a resource on the internet. Although people often think of a URL as a single opaque address, it is actually composed of distinct components. By splitting the address into parts, developers can manipulate or validate each piece individually, making it easier to build tools, diagnose issues, or construct dynamic links.

The general layout of an HTTP URL can be expressed using the concatenation formula:

In this expression, the scheme indicates the protocol such as https or ftp; the authority contains optional user information, a host name, and an optional port; the path points to a resource on the server; the query holds key-value pairs for application parameters; and the fragment references a subsection of the resulting document. Not every URL contains all parts, but the order of those that appear must follow the pattern.

Parsing URLs matters because tiny mistakes can lead to broken links or security vulnerabilities. Encoding issues in the path or query string might cause a server to misinterpret characters. Missing slashes can direct users to the wrong directory. When applications accept user-supplied URLs, validating the scheme and host helps prevent malicious redirects. By examining each component separately, one can ensure that only expected values are processed.

The table below outlines the primary properties returned by the browser's URL interface:

Property	Description
protocol	The scheme including the trailing colon.
username	User name specified before the host, if any.
password	Password associated with the username, rarely used on modern sites.
hostname	Domain name or IP address without port.
port	Port number after the colon, blank if default.
pathname	Path starting with a slash.
search	Query string including the leading question mark.
hash	Fragment identifier including the hash sign.
origin	Scheme, host, and port combined.

When parameters appear in the query string, each takes the form key=value separated by ampersands. The URLSearchParams interface parses this section into a map. This tool lists those pairs in a separate table so that you can see exactly what the browser interprets from the original string. If the same key occurs multiple times, URLSearchParams preserves the order and exposes methods like getAll to handle duplicates.

Consider a real-world example: https://user:pass@example.com:8443/catalog/item?color=red&size=medium#details. Parsing yields a protocol of https:, a username of user, a password of pass, a hostname of example.com, a port of 8443, a pathname of /catalog/item, a query string of ?color=red&size=medium, and a fragment of #details. Two parameters, color and size, become immediately accessible without manual string slicing.

Understanding URLs also assists with search engine optimization and analytics. Marketing teams often append UTM parameters like ?utm_source=newsletter to track campaigns. Developers may need to remove or rewrite these parameters before storing or forwarding the link. A parser lets you inspect such metadata quickly to verify that your links carry the correct tags.

Security is another reason to parse URLs carefully. Attackers sometimes craft links that visually resemble a trusted domain but actually redirect to malicious sites using techniques like internationalized domain names. By comparing the hostname property to a whitelist of allowed hosts, applications can detect suspicious URLs before navigation occurs. Additionally, stripping unexpected schemes prevents the execution of javascript: or data: URLs in contexts where only http or https should be permitted.

URL parsing is not limited to web browsers. Many programming languages provide libraries to decompose URLs for server-side operations. In shell scripts, parsing ensures that automated downloads target the right resources. Configuration files often embed URLs for connecting to databases or APIs; validating the host and port reduces configuration mistakes. The widespread need for parsing makes this utility a handy reference during development.

The formula for converting a set of discrete parameters into a query string can be expressed succinctly as:

This representation, while simplified, emphasizes the pairing of keys and values. Encoding special characters requires percent-encoding based on their byte values. For example, spaces become %20 or plus signs depending on context. Our parser does not modify the encoding but reveals exactly what the browser perceives.

The following table provides sample URLs illustrating different combinations of components:

Example URL	Notes
https://example.com	Basic URL with only scheme and host.
ftp://user@example.com:21/docs	Includes username and explicit port.
https://shop.example.com/products?id=5#reviews	Contains subdomain, path, query, and fragment.
file:///C:/Windows/System32/	File scheme without host.

By experimenting with these examples in the parser above, you can observe how each part contributes to the overall address. This deep understanding becomes invaluable when constructing APIs, designing routing rules, or performing migrations between domains. Instead of treating URLs as opaque strings, you gain precise control over their structure.

Historically, the modern URL format was standardized by Tim Berners-Lee and the Internet Engineering Task Force in RFC 1738 and later refined in RFC 3986. The use of a hierarchical path and query parameters allowed the early web to map resources without specifying transport details. Over time, new schemes like mailto: or tel: expanded the concept beyond HTTP, enabling links to compose emails or initiate phone calls. Despite these variations, the essential grammar remains recognizable, and this parser follows the formal grammar defined in RFC 3986.

In summary, parsing URLs is fundamental for anyone working with web technologies. This tool demonstrates how the browser's native API exposes each component, empowering you to validate inputs, debug routing, and manipulate parameters with confidence. Because the logic executes locally in your browser, you can experiment with sensitive URLs without transmitting them to a server. The more comfortable you become with dissecting addresses, the more reliably you can design systems that handle them.