URL Encoding (Percent-encoding)

URL Encoding, officially known as percent-encoding, is a mechanism for encoding information in a Uniform Resource Identifier (URI). It is used to ensure that characters that have special meaning in a URL (like ?, &, =, #) or characters that are not permitted in URLs (like spaces or non-ASCII characters) are safely transmitted over the internet.

How it works

When a character needs to be URL encoded, it is replaced by a percent sign (%) followed by the two-digit hexadecimal representation of its ASCII (or UTF-8) value.

For example:

• A space ( ) becomes %20 (or sometimes + in form data)
• An exclamation mark (!) becomes %21
• A question mark (?) becomes %3F
• An ampersand (&) becomes %26

Reserved vs. Unreserved Characters

The URI specification defines two types of characters:

1. Unreserved Characters: Characters that are allowed in a URL and do not need to be encoded. These include uppercase and lowercase letters (A-Z, a-z), decimal digits (0-9), hyphen (-), period (.), underscore (_), and tilde (~).
2. Reserved Characters: Characters that have special meaning within the URL syntax (such as acting as delimiters). If data being passed in a URL contains these characters, they must be percent-encoded to distinguish them from their role as delimiters. Reserved characters include ! * ' ( ) ; : @ & = + $ , / ? % # [ ].

Common Use Cases

• Query Parameters: When submitting HTML forms using the GET method, the form data is appended to the URL as a query string. Spaces and special characters in user input must be URL encoded.
• API Requests: REST APIs often accept data via URL parameters or path variables, requiring the client to URL encode the data before sending the request.