Redis keys are binary-safe, so literally anything can be used as a key. The only limitations are that they must be less than 512MB.
Examples of valid keys:
7 ++++ `~!@#$%^&*()-_=+ user:10134 search/9947372/?query=this%20is%20a%28test%29%20query <div id="div64"> Any other string less than 512MB in size. The raw binary content of an image or other binary file. An entire multi-line text document. An entire SQL query. Any integer, hexadecimal, octal, or binary value. Anything else you can think of less than 512MB in size.
Invalid Redis keys:
Anything larger than 512MB.