What are the user defined key's value restrictions?

Quoting from the manual

• The key must be a string value. Numeric keys are not allowed, but any numeric value can be put into a string and can then be used as document key.
• The key must be at least 1 byte and at most 254 bytes long. Empty keys are disallowed when specified (though it may be valid to completely omit the _key attribute from a document)
• It must consist of the letters a-z (lower or upper case), the digits 0-9 or any of the following punctuation characters: _ - : . @ ( ) + , = ; $ ! * ' %
• Any other characters, especially multi-byte UTF-8 sequences, whitespace or punctuation characters cannot be used inside key values
• The key must be unique within the collection it is used

Keys are case-sensitive, i.e. myKey and MyKEY are considered to be different keys.