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Percent-encoding unreserved characters

Characters from the unreserved set can be percent-encoded in the same way as reserved characters. That is, if a scheme calls for an unreserved character to be used in a URI, either the raw character or its percent-encoded equivalent may be used interchangeably.

URIs that differ only by the percent-encoding of unreserved characters are equivalent by definition, but URI processors, in practice, may not always recognize this equivalence. For example, URI consumers shouldn't treat "%41" differently than "A" or "%7E" differently than "~", but some do. For maximum interoperability, URI producers are discouraged from percent-encoding unreserved characters.

Percent-encoding arbitrary data

Most URI schemes involve the representation of arbitrary data, such as an IP address or file system path, as components of a URI. URI scheme specifications should, but often don't, provide an explicit, clear mapping between URI characters and all possible data values being represented by those characters.

Binary data

Since the publication of RFC 1738 in 1994, it has been specified that schemes that provide for the representation of binary data in a URI must divide the data into 8-bit bytes and percent-encode each byte in the same manner as above, being careful to use raw unreserved characters, rather than percent-encoded sequences, where possible. For example, byte value FF (hexadecimal) should be represented by "%FF", but byte value 41 (hexadecimal) should be represented by "A", not "%41".

Character data

The procedure for percent-encoding binary data has often been extrapolated, sometimes inappropriately or without being fully specified, to apply to character-based data. In the World Wide Web's formative years, when dealing with data characters in the ASCII repertoire and using their corresponding bytes in ASCII as the basis for determining percent-encoded sequences, this practice was relatively harmless; it was just assumed that characters and bytes mapped one-to-one and were interchangeable. However, the need to represent characters outside the ASCII range quickly grew, and URI schemes and protocols often failed to provide standard rules for preparing character data for inclusion in a URI. Consequently, web applications began using different multi-byte, stateful, and other non-ASCII-compatible encodings as the basis for percent-encoding, leading to ambiguities and difficulty interpreting URIs reliably.

For example, many URI schemes and protocols based on RFCs 1738 and 2396 presume that the data characters will be converted to bytes according to some unspecified character encoding before being represented in a URI by unreserved characters or percent-encoded bytes. If the scheme does not allow the URI to provide a hint as to what encoding was used, or if the encoding conflicts with the use of ASCII to percent-encode reserved and unreserved characters, then the URI cannot be reliably interpreted. Some schemes fail to account for encoding at all, and instead just suggest that data characters map directly to URI characters, which leaves it up to implementations to decide whether and how to percent-encode data characters that are in neither the reserved nor unreserved sets.

Current standard

The generic URI syntax mandates that new URI schemes that provide for the representation of character data in a URI must, in effect, represent characters from the unreserved set without translation, and should convert all other characters to bytes according to UTF-8, and then percent-encode those values. This requirement was introduced in January 2005 with the publication of RFC 3986. URI schemes introduced before this date are not affected.

Not addressed by the current specification is what to do with encoded character data. For example, in computers, character data manifests in encoded form, at some level, and thus could be treated as either binary data or as character data when being mapped to URI characters. Presumably, it is up to the URI scheme specifications to account for this possibility and require one or the other, but in practice, few, if any, actually do.

Non-standard implementations

Internet Explorer uses a non-standard encoding for Unicode characters: %uxxxx, where xxxx is a Unicode value represented as four hexadecimal digits. This behavior is not specified by any RFC and has been rejected by the W3C.

The application/x-www-form-urlencoded type

When data that has been entered into HTML forms is submitted, the form field names and values are encoded and sent to the server in an HTTP request message using method GET or POST, or via email. The encoding used by default is based on a very early version of the general URI percent-encoding rules, with a number of modifications such as newline normalization and replacing spaces with "+" instead of "%20". The MIME type of data encoded this way is application/x-www-form-urlencoded, and it is currently defined (still in a very outdated manner) in the HTML and XForms specifications. In addition, the CGI specification contains rules for how web servers decode data of this type and make it available to applications.

When sent in an HTTP GET request, application/x-www-form-urlencoded data is included in the query component of the request URI. When sent in an HTTP POST request or via email, the data is placed in the body of the message, and the name of the media type is included in the message's Content-Type header.

External links

The following specifications all discuss and define reserved characters, unreserved characters, and percent-encoding, in some form or other:

• RFC 3986 / STD 66, the current generic URI syntax specification.
• RFC 2396 (obsolete) and RFC 2732 together comprised the previous version of the generic URI syntax specification.
• RFC 1738 (mostly obsolete) and RFC 1808 (obsolete), which define URLs.
• RFC 1630 (obsolete), the first generic URI syntax specification.
• W3C Guidelines on Naming and Addressing: URIs, URLs, ...
• W3C explanation of UTF-8 in URIs

Other resources:

• URL Encoding (or: 'What are those "%20" codes in URLs?') by Brian Wilson – URL encoding charts and a converter
• URLEncode Code Chart (from i-Technica) – a URL encoding chart
• Online Tool for URL Encoding/Decoding