An expression is a value (a constant, a predefined constant, a variable or the result of a function), which may optionally be preceded by certain operators depending on the type of value, followed by an operator and another value, and so on.

For numeric expressions:
 { number | constant | numeric function | numeric variable } [ operator [ sign ] { number | numeric function | numeric variable} ... ]
  Number : a standard Number
  constant : a predefined constant that represents a numeric value
  numeric function : a user-defined or Intrinsic Functions that returns a numeric value
  operator : any of the standard arithmetic operators ( + - * / ^ \  DIV MOD plus AND OR or XOR)
  numeric variable : any variable whose value is a number

For string expressions:
{ string | string constant | string function | string variable} [ & { string | string constant | string function | string variable} ... ]
  string : a set of zero or more characters surrounded by "
  string constant : a predefined constant that represents a string value
  string function : a user-defined or Intrinsic Functions that returns a string value
  string variable : any variable whose value is a string
  & : indicates that the string value to the right of the ampersand is to be concatenated to the end of the value to the right of the ampersand

For boolean expressions:
[ NOT ] { TRUE | FALSE | numeric constant | boolean constant | boolean function | boolean variable } [ boolean operator [ NOT ] { TRUE | FALSE | numeric constant | boolean constant | boolean function | boolean variable } ... ]
  NOT : using NOT before a boolean value reverses the value
  TRUE or FALSE : the standard boolean constants
  numeric constant : a standard number which will be resolved to either TRUE or FALSE[1]
  boolean constant : a predefined constant that represents a boolean value
  boolean function : a user defined or Intrinsic Functions that returns  TRUE or FALSE or resolves to  TRUE or FALSE[1]
  boolean variable : any variable whose value is TRUE or FALSE or resolves either to TRUE or FALSE[1]
  boolean operator: AND OR XOR = <> < > <= >=
Note that if the expression consists of a number with the operator AND, OR, or XOR followed by a number, it will be treated as an arithmetic expression rather than a boolean expression.  At least one of the values in the expression must be a boolean value to produce a boolean expression.

[1]Note that a Number, predefined constant, arithmetic function or numeric variable may be used as a boolean value.  If the value of a number, numeric constant, numeric function, or variable is used in a boolean expression is zero, it resolves to FALSE ; all other numeric values used in a boolean expression resolve to TRUE.