The metadata of a zettel is a collection of key-value pairs. The syntax roughly resembles the internal header of an email (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:
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.