Rules for coding expressions

The rules for coding an absolute or relocatable expression are:

Unary (operating on one value) operators and binary (operating on two values) operators are allowed in expressions.
An expression can have one or more unary operators preceding any term in the expression or at the beginning of the expression.
An expression must not begin with a binary operator, nor can it contain two binary operators in succession. When + and - are used as prefix operators, then they are unary, and not binary, operators.
An expression starting with * is interpreted as a location counter reference, and not a multiplication operator.
An expression must not contain two terms in succession.
No spaces are allowed between an operator and a term, nor between two successive operators.
An expression can contain any number of unary and binary operators, and any number of levels of parentheses.
A single relocatable term is not allowed in a multiply or divide operation. Paired relocatable terms have absolute values and can be multiplied and divided if they are enclosed in parentheses. See Paired relocatable terms.

Figure 1 shows the definitions of absolute and relocatable expressions.

Figure 1. Definitions of absolute and relocatable expressions

┌───────────┐
│           │
│Absolute   │
│Expression │
│           │
└─────┬─────┘
      │
      V Can be any of
      ├─────────────┬─────────────┬─────────────┬─────────────┬─────────────┬─────────────┬─────────────┬─────────────┐
      │             │             │             │             │             │             │             │             │
      V             V             V             V             V             V             V             V             V
┌─────┴─────┐ ┌─────┴─────┐ ┌─────┴─────┐ ┌─────┴─────┐ ┌─────┴─────┐ ┌─────┴─────┐ ┌─────┴─────┐ ┌─────┴─────┐ ┌─────┴─────┐
│           │ │           │ │           │ │           │ │           │ │           │ │           │ │           │ │           │
│ Rel. Exp. │ │ Absolute  │ │ Abs. Exp. │ │ Abs. Exp. │ │ Abs. Exp. │ │ Abs. Exp. │ │(Abs. Exp.)│ │ +Abs. Exp.│ │ -Abs. Exp.│
│     -     │ │ Term      │ │     +     │ │     -     │ │     *     │ │     /     │ │           │ │ ∧         │ │ ∧         │
│ Rel. Exp. │ │           │ │ Abs. Exp. │ │ Abs. Exp. │ │ Abs. Exp. │ │ Abs. Exp. │ │           │ │ │         │ │ │         │
└─────┬─────┘ └─────┬─────┘ └───────────┘ └───────────┘ └───────────┘ └───────────┘ └───────────┘ └─┼─────────┘ └─┼─────────┘
      │             │                                                                               └──────┬──────┘
      │             V Can be any of                                                                        │
      │             ├─────────────┬─────────────┬─────────────┬─────────────┐                          unary operators
      │             │             │             │             │             │
      │             V             V             V             V             V
      │       ┌─────┴─────┐ ┌─────┴─────┐ ┌─────┴─────┐ ┌─────┴─────┐ ┌─────┴─────┐
      │       │Absolute   │ │ Self-     │ │ Symbol    │ │ Symbol    │ │ Symbol    │
      │       │Valued     │ │ Defining  │ │ Length    │ │ Integer   │ │ Scale     │
      │       │Ordinary   │ │ Term      │ │ Attribute │ │ Attribute │ │ Attribute │
      │       │Symbol     │ │           │ │           │ │           │ │           │
      │       └───────────┘ └───────────┘ └───────────┘ └───────────┘ └───────────┘
      │
      V
┌─────┴─────┐
│           │
│Relocatable│
│Expression │
│           │
└─────┬─────┘
      │
      V Can be any of
      ├─────────────┬─────────────┬─────────────┬─────────────┬─────────────┐
      │             │             │             │             │             │
      V             V             V             V             V             V
┌─────┴─────┐ ┌─────┴─────┐ ┌─────┴─────┐ ┌─────┴─────┐ ┌─────┴─────┐ ┌─────┴─────┐           Operators Allowed
│           │ │           │ │           │ │           │ │           │ │           │
│Relocatable│ │ Rel. Exp. │ │ Rel. Exp. │ │(Rel. Exp.)│ │+Rel. Exp. │ │-Rel. Exp. │           Unary:  + Positive
│Term       │ │     +     │ │     -     │ │           │ │∧          │ │∧          │                   - Negative
│           │ │ Abs. Exp. │ │ Abs. Exp. │ │           │ ││          │ ││          │
└─────┬─────┘ └───────────┘ └───────────┘ └───────────┘ └┼──────────┘ └┼──────────┘           Binary: + Addition
      │                                                  │             │                              - Subtraction
      │                                                  └──────┬──────┘                              * Multiplication
      V Can be any of                                           │                                     / Division
      ├─────────────┐                                    unary operators
      │             │
      V             V
┌─────┴─────┐ ┌─────┴─────┐
│Relocatable│ │ Location  │                                                                   Rel. Exp. = Relocatable Expression
│Valued     │ │ Counter   │                                                                   Abs. Exp. = Absolute Expression
│Ordinary   │ │ Reference │
│Symbol     │ │           │
└───────────┘ └───────────┘