RFC 1738: Uniform Resource Locators (URL) specification
The specification for URLs (RFC 1738,
Dec. '94) poses a problem, in that it limits the use of allowed characters
in URLs to only a limited subset of the US-ASCII character set:
"...Only alphanumerics [0-9a-zA-Z],
the special characters "$-_.+!*'()," [not including the quotes -
ed], and reserved characters used for their reserved purposes may
be used unencoded within a URL."
HTML, on the other hand, allows the entire range of the
ISO-8859-1 (ISO-Latin) character set
to be used in documents - and HTML4 expands the allowable range to include
all of the Unicode character
set as well.
What characters need to be encoded and why?
| "Reserved characters" |
| |
Why: |
URLs use some characters for special use in defining their syntax.
When these characters are not used in their special role inside a
URL, they need to be encoded. |
| Characters: |
";", "/", "?", ":", "@", "=" and "&" |
| ASCII Control characters |
| |
Why: |
These characters are not printable. |
| Characters: |
Includes the ISO-8859-1
(ISO-Latin) character ranges 00-1F hex (0-31 decimal) and 7F
(127 decimal.) |
| Non-ASCII characters |
| |
Why: |
These are by definition not legal in URLs since they are not in
the ASCII set. |
| Characters: |
Includes the entire "top half" of the
ISO-Latin set 80-FF hex
(128-255 decimal.) |
| "Unsafe characters" |
| |
Why: |
Some characters present the possibility of not being
understood within URLs for various reasons. These characters
should also always be encoded. |
| Characters: |
| Character | Code
Points
(Hex) |
Code
Points
(Dec) | Why encode? |
|---|
| Space | 20 | 32 |
Significant sequences of spaces may be lost in some
uses (especially multiple spaces) |
Quotation marks
'Less Than' symbol ("<")
'Greater Than' symbol (">") |
22
3C
3E | 34
60
62 |
These characters are often used to delimit URLs
in plain text. |
| 'Pound' character ("#") |
23 | 35 |
This is used in URLs to indicate where a fragment
identifier (bookmarks/anchors in HTML) begins. |
| Percent character ("%") |
25 | 37 |
This is used to URL encode/escape other characters,
so it should itself also be encoded. |
Misc. characters:
Left Curly Brace ("{")
Right Curly Brace ("}")
Vertical Bar/Pipe ("|")
Backslash ("\")
Caret ("^")
Tilde ("~")
Left Square Bracket ("[")
Right Square Bracket ("]")
Grave Accent ("`") |
7B
7D
7C
5C
5E
7E
5B
5D
60 |
123
125
124
92
94
126
91
93
96 |
Some systems can possibly modify these characters. |
|
How are characters URL encoded?
URL encoding of a character consists of a "%" symbol,
followed by the two-digit hexadecimal representation (case-insensitive)
of the ISO-Latin code point for the
character.
- Example
- Space = decimal code point 32 in the
ISO-Latin set.
- 32 decimal = 20 in hexadecimal
- The URL encoded representation will be "%20"
URL encode your own characters
Type any character into the "Original Character" box
below and you can easily convert it to the URL encoded representation of
the character.
Browser Peculiarities
- Internet Explorer is notoriously relaxed in its requirements for
encoding spaces in URLs. This tends to contribute to author
sloppiness in authoring URLs. Keep in mind that Netscape and
Opera are much more strict on this point, and spaces MUST
be encoded if the URL is to be considered to be correct.
|