Question 1

What is percent encoding?

Accepted Answer

Percent encoding is the standard mechanism for encoding characters in URIs, defined by RFC 3986. Characters that are not allowed in a URI component are represented as %XX, where XX is the hexadecimal value of each byte. For multi-byte UTF-8 characters, each byte is separately percent-encoded (e.g., the Chinese character for "good" becomes %E5%A5%BD).

Question 2

What is the difference between RFC 3986 and RFC 2396?

Accepted Answer

RFC 3986 (2005) obsoletes RFC 2396 (1998). The main difference is that RFC 3986 removed the distinction between "mark" and "unreserved" characters, simplified the unreserved set to A-Z a-z 0-9 - _ . ~, and made tilde (~) unreserved instead of reserved. RFC 3986 also updated the definition of reserved characters and clarified when percent-encoding is required.

Question 3

Why is space encoded as %20 and not +?

Accepted Answer

In RFC 3986 percent encoding, space is always encoded as %20. The + representation for spaces comes from the application/x-www-form-urlencoded format used in HTML forms, which is a separate encoding standard. While + for space is valid in query strings submitted by HTML forms, %20 is the universal standard that works everywhere in URIs. The encodeURIComponent function uses %20, not +.

Character	Encoded Form	Purpose in URI
:	%3A	Scheme/port separator (http:, :8080)
/	%2F	Path segment separator (/path/to)
?	%3F	Query string start (?key=value)
#	%23	Fragment identifier (#section)
[	%5B	IPv6 address bracket
]	%5D	IPv6 address bracket
@	%40	User info separator (user@host)
!	%21	Sub-delimiter
$	%24	Sub-delimiter
&	%26	Query parameter separator (&key=value)
'	%27	Sub-delimiter
(	%28	Sub-delimiter
)	%29	Sub-delimiter
*	%2A	Sub-delimiter
+	%2B	Sub-delimiter (also space in forms)
,	%2C	Sub-delimiter
;	%3B	Sub-delimiter (path parameters)
=	%3D	Key-value separator (key=value)

Character	Encoded Form	Purpose in URI
space	%20	Not allowed in URIs
"	%22	Not allowed (delimiter in HTML)
<	%3C	Not allowed (HTML tag)
>	%3E	Not allowed (HTML tag)
{	%7B	Not allowed (URI template)
}	%7D	Not allowed (URI template)
\|	%7C	Not allowed
\	%5C	Not allowed (escape char)
^	%5E	Not allowed
`	%60	Not allowed
%	%25	Percent sign itself (to avoid ambiguity)

Percent Encoding Tool

RFC 3986 Percent Encoding Reference

Unreserved Characters (never need encoding)

Reserved Characters (have special meaning in URIs)

Must-Encode Characters (common examples)

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