The metadata of a zettel is a collection of key-value pairs.
The syntax roughly resembles the internal header of an email ([[RFC5322|https://tools.ietf.org/html/rfc5322]]).

The key is a sequence of alphanumeric characters, a hyphen-minus character (""''-''"", U+002D) is also allowed.
It begins at the first position of a new line.
Uppercase letters of a key are translated to their lowercase equivalence.

A key is separated from its value either by
* a colon character (""'':''""),
* a non-empty sequence of space characters,
* a sequence of space characters, followed by a colon, followed by a sequence of space characters.
A value is a sequence of printable characters.
If the value should be continued in the following line, that following line (""continuation line"") must begin with a non-empty sequence of space characters.
The rest of the following line will be interpreted as the next part of the value.
There can be more than one continuation line for a value.

A non-continuation line that contains a possibly empty sequence of characters, followed by the percent sign character (""''%''"") is treated as a comment line.
It will be ignored.

Parsing metadata ends, if an empty line is found or if a line with at least three hyphen-minus characters is found.

Some examples:
```{="meta"}
title1:The Title
 title-2 : Another title
title-3: A wrapped
 title
title-4: A
 wrapped
 title
 with
 more
 than
 one
  continuation
 line
% A comment line
 % Another comment line.

No metadata anymore, because of the empty line.
```