Percentencode

Percent-encode, or percent-encoding, is a mechanism for representing arbitrary data in a URI by replacing certain bytes with a percent sign followed by two hexadecimal digits. It is widely used to ensure that data conforms to the allowed character set of URLs and other URIs.

In practice, text is first encoded into a byte sequence (UTF-8 is common for Unicode). Any byte that is not an unreserved character is encoded as %HH, where HH are two hexadecimal digits. The unreserved set consists of A–Z, a–z, 0–9, and the characters -, ., _, ~. The exact encoding rules can vary by URI component; unreserved characters may be left unencoded, while other characters should be percent-encoded to prevent misinterpretation of the URI’s syntax.

Common examples include encoding a space as %20, a forward slash as %2F, and non‑ASCII characters such as é as %C3%A9 in UTF-8. Decoding is the reverse operation: percent-encoded bytes are converted back to their original bytes and then interpreted with the appropriate character encoding.

Percent-encoding is defined in RFC 3986 and is essential for safely transmitting data in URLs, including path segments, query strings, and fragments. There are practical variations in form data submission (application/x-www-form-urlencoded often uses '+' to represent spaces). Most programming languages provide libraries or built-in functions to perform encoding and decoding, such as encodeURIComponent and encodeURI in JavaScript or urllib.parse in Python.