# XN Syntax

## Comments

Comments are single line only prefixed with the pound sign ('#').  

## Rules

Rules consists of two parts, a selector, and a block containing statements.  
A selector is an XPath string surrounded with braces prefixed with a dollar sign.  
The XPath selects zero or more document nodes which will serve as the context for the statement block.  

## Bangs

Bangs are function calls combined with a statement block.  
They are prefixed with an exclaimation mark.  
A bang function always produces context nodes for the statement block.  

## Statement Block
A statement block are multiple statements terminated with a semicolon,  
surrounded by curly braces.

## Statements

A statement can be an expression, a function call, a rule or a bang.  
To note is that a nested rule will use its parents context to refine the selection further.  

## Expressions

An expression can consist of function calls, operators, strings or numbers.

Operators supported:

| Operator | Function             |
|----------|----------------------|
| +        | addition             |
| -        | subtraction          |
| *        | multiplication       |
| /        | division             |
| **       | power                |
| &        | bitwise and          |
| |        | bitwise or           |
| ^        | bitwise exclusive or |
| ..       | string concatenate   |

## Example

```
# Select the RxPdo with Index 0x1600
$("RxPdo/Index[.="#x1600"]/..")
{
    # Function call
    swap("Entry[3]", "Entry[4]");

    # Of that RxPdo, select the 3rd Entry via a nested rule
    $("Entry[3]")
    {
        # This bang has "RxPdo/Index[.="#x1600"]/../Entry[3]" as its context
        !before("Entry")
        {
             # Function call contains an expression, which evaluates here to "!#xFFFF"
            !add("Index") { text("!"..hexlit(2**16-1)); };
            !add("SubIndex") { text("3"); }
            !add("BitLen") { text("256"); }
            !add("Name") { text("Test"); }
            !add("DataType") { text("UINT"); }
        }
    }
}

# One .xn can contain multiple rules
$("//Module[@Crc32]")
{
    attr("Crc32", hexlit(crc32()));
};
```