Some characters cannot be part of a URL (for example, the space) and some other characters have a special meaning in a URL: for example, the character # is used to locate a point within a page; the character = is used to separate a name from a value. A query string may need to be converted to satisfy these constraints. This can be done using a schema known as URL encoding.

In particular, encoding the query string uses the following rules:

• Letters (A-Z and a-z), numbers (0-9) and the characters '.', '-', '~' and '_' are left as-is
• SPACE is encoded as '+'
• All other characters are encoded as %FF hex representation with any non-ASCII characters first encoded as UTF-8 (or other specified encoding)

The encoding of SPACE as '+' and the selection of "as-is" characters distinguishes this encoding from RFC 1738.