The documentation states:
Characters ?{}|&~!()^" must not be used anywhere in the key and have a special meaning in the value.
Here's the results of my experiments on what they mean:
; | is used for bitwise OR
three = 2|3
; & is used for bitwise AND
four = 6&5
; ^ is used for bitwise XOR
five = 3^6
; ~ is used for bitwise negate
negative_two = ~1
; () is used for grouping
seven = (8|7)&(6|5)
; ${...} is used for grabbing values from the environment, or previously defined values.
path = ${PATH}
also = ${five}
; ? I have no guess for
; ! I have no guess for