percentencoding

Percent-encoding, also known as URL encoding, is a mechanism for representing arbitrary data within a URL by replacing unsafe characters with a percent sign followed by two hexadecimal digits. It is defined by RFC 3986 and related specifications and is used to include characters that are not allowed in a URI component, such as spaces, punctuation in certain contexts, and non-ASCII text.

How it works: there is a set of unreserved characters that may appear without encoding: uppercase and lowercase letters, digits, hyphen, underscore, period, and tilde. All other characters must be encoded. For non-ASCII text, the string is first encoded in UTF-8, and then each resulting byte is represented by a percent sign and its two-digit hexadecimal value (for example, a byte value of 32 becomes %20).

Context matters: path segments, query components, and other URI parts have slightly different rules. The query form encoding widely used on the web, application/x-www-form-urlencoded, replaces spaces with a plus sign (+) rather than %20, and may apply percent-encoding to other characters. In most programming languages, libraries such as encodeURIComponent/encodeURI (JavaScript) or urllib.parse.quote (Python) automate percent-encoding and decoding.

Practical considerations include avoiding double encoding, ensuring consistent UTF-8 input, and decoding on the server side. Understanding the exact rules for the target component helps avoid ambiguous or invalid URLs.