Checks if a value is a valid Selection entry key.
The value to check
True if value is a string composed of a Selection operator prefix (<, >, <=, >=, ~, ?, !, +, ^) followed by a valid Expression, or exactly "@" or "#"; false otherwise
value
<
>
<=
>=
~
?
!
+
^
"@"
"#"
Checks if a value is a valid Selection entry key.