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Precedence of pattern operations
Prerequisite Concepts

Parentheses must be used to change the precedence of an operation. The following list shows the precedence of all pattern operations when there are no parentheses, starting with the highest precedence:

1. ul
2. optionality, repetition, and general occurrence operators ("?", "*", "+", and "{"..."}")
3. entity-type prefixes (cdata, text, etc)
4. pattern-variable assignments (using =>)
5. sequences (a pattern followed by a pattern, etc)
6. lookahead and lookahead ! (lookahead not)
7. | (or)

When a lower-precedence operation is embedded within a higher-precedence operation, the precedence of pattern operations requires parentheses, as follows:

• letter => id ? is not allowed, and must be entered as (letter => id)?.
• lookahead letter | digit ! digit is not allowed, and must be entered as lookahead (letter | digit) ! digit.

This code sample illustrates the precedence of pattern operations:

  translate "%n" digit => key-digit
            lookahead text letter+ space ! ul "key" "word"? |
            sdata "[sect]" lookahead "." ! " " |
            digit+ => number

  translate (("%n" (digit => key-digit))
             lookahead ((text (letter+)) space)
                ! ((ul "key") ("word"?))) |
            ((sdata "[sect]") lookahead "." ! " ") |
            ((digit+) => number)

Prerequisite Concepts      Pattern matching

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