percentencoded

Percent-encoded, commonly called URL encoding, is a mechanism for representing arbitrary octets in URIs by replacing them with a percent sign followed by two hexadecimal digits. It is defined by RFC 3986 as a way to encode data that would otherwise be interpreted as reserved or disallowed characters in a URI.

It typically applies to the bytes of the string encoded in UTF-8. Therefore, characters outside the unreserved set are converted to percent-encoded form. The unreserved characters are A–Z, a–z, 0–9, and the characters dash, period, underscore, and tilde; all others should be encoded in a URI component.

Each percent-encoded element represents one byte, as %HH where HH is the hexadecimal value of that byte. For example, the space character is encoded as %20; the colon ":" as %3A; and the euro sign € as %E2%82%AC when the source text is UTF-8. Hex digits are typically written in uppercase, though lowercase is valid.

The encoding is used in all parts of a URI, including the path and the query string, to allow data that would otherwise alter the structure of the URI to be transmitted safely. Practitioners should avoid double encoding and be aware of differences between percent-encoding in URIs and application/x-www-form-urlencoded in HTML forms.

Related topics include URIs, RFC 3986, URL encoding, and UTF-8.