LOL! Who knew that `:` was a valid function name?

It is not a valid variable name in Bash though. I think it should be a valid fork bomb even without the pipe.

Yes, the POSIX specification seems to allow implementations to use non-alphanumeric characters in function names too. Quoting the relevant paragraph from POSIX.1-2001 specification:

> The function is named fname; the application shall ensure that it is a name (see the Base Definitions volume of IEEE Std 1003.1-2001, Section 3.230, Name). An implementation may allow other characters in a function name as an extension. The implementation shall maintain separate name spaces for functions and variables.