The standard for internet addresses RFC 3986 allows only certain charatcers to be part of an URL: The 26 basic latin characters in small and capital letters, digits, hyphen, underscore, dot and tilde:
a-z A-Z 0-9 - _ . ~

All other characters are part of the "reserved space" of the RFC 3986. These characters either have a special function in an URL such as dash "/" and colon ":", or they are simply not part of the URL namespace. To submit these characters in an URL, they are converted into a special format called URL encoding or percentage encoding. Instead of the character itself, its position in the Ascii charset is given, preceded by a percentage sign. Thus, a space " " will turn into %20, and the umlaut ä into %E4.

Here is a list of the typical characters that can be found in percent-encoded URLs and "garbled" E-Mails: