URL Encoding

URL Syntax

https://admin:pass123@www.example.com:80/bio.txt;pp=1&qp=2#Three

URL Part URL Data
Scheme https
User admin
Password pass123
Subdomain www
Domain example.com
Port 80
Path /bio.txt
Path Parameter pp=1
Query Parameter qp=2
Fragment Three

Safe Characters

RFC1738 section 2.2 outlines the safe characters to use in an HTTP URL Scheme:

abcdefghijklmnopqrstuvwxyz0123456789$-_.+!*'(),

Safe characters can be used in URLs without any form of encoding as they aren’t reserved for special use in the construction of the URL.

Unsafe Characters

Per RFC1738 section 2.2, the following characters are unsafe for use in an HTTP URL Scheme:

space < > " # % { } | \ ^ ~ [ ] `

RFC1738 section 2.2 also states that the following characters are reserved in an HTTP URL Scheme:

; / ? : @ = &

RFC3986 section 2.2 additionally specifies reserved characters in URI schemes:

space % : / ? # [ ] @ ! $ & ' ( ) * + , ; =

Unsafe and reserved characters are reserved for use in constructing the URL scheme. These characters must be encoded so the URL can be constructed without ambiguity. Fortunately, RFC1738 has us covered.

URL Encoding

URL, or percent, encoding substitutes the percent (%) sign and two hexadecimal characters to represent unsafe characters in a URL. Here are the encodings for unsafe and reserved characters per RFCs 1738 and 3986:

Unsafe Character URL(Percent) Encoding
space %20
% %25
: %3A
/ %2F
? %3F
# %23
[ %5B
] %5D
@ %40
! %21
$ %24
& %26
%27
( %28
) %29
* %2A
+ %2B
, %2C
; %3B
= %3D
< %3C
> %3E
%22
{ %7B
} %7D
pipe %7C
%5C
^ %5E
~ %7E
` %60

URL ENCODING “GOTCHAS”

Stéphane Épardaud describes several pitfalls to URL encoding at the Lunatech Blog (go read his post!). In summary, reserved characters differ for each part of the URL:

Path

spaces must be encoded to %20, not + : @ - . _ ~ ! $ & grave ( ) * + , ; = are allowed unencoded

Path Parameter

= is allowed unencoded

Query

spaces may be encoded to + (for backward compatibility) or %20. + must be encoded to %2B ? , / are allowed unencoded

Fragment

/ ? : @ - . _ ~ ! $ & grave ( ) * + , ; = are allowed unencoded

For more information

RFC1738 - Uniform Resource Locators
RFC3986 - Uniform Resource Identifiers
What every web developer must know about URL encoding

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