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# Syntax Reference

This is the official specification of the blueprint format.

The grammar is expressed as a parsing expression grammar. This has two important implications: the parser will never backtrack, and alternation (e.g. a `|` in the specification) will always take the first branch that matches, even if that causes an error later. These properties make PEGs both unambiguous and simple to implement in code.

Blueprint uses C-style line comments (`// comment for the rest of the line`) and block comments (`/* multiline comment... */`).

Wherever commas are used as delimiters in repetition (expressed in this reference as `( <rule> ),*`), the trailing comma is permitted and optional.

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## Tokens

### IDENT

An identifier starts with an ASCII underscore `_` or letter `[A-Za-z]` and consists of ASCII underscores, letters, digits `[0-9]`, and dashes `-`. Dashes are included for historical reasons, since GObject properties are traditionally kebab-case.

### NUMBER

Numbers begin with an ASCII digit and consist of ASCII digits, underscores, dots `.`, and letters (for radix pre-/suffixes). More than one dot in a number is not allowed. Underscores are permitted for increased readability, and are ignored.

Hexadecimal numbers may be specified using the `0x` prefix and may use uppercase or lowercase letters, or a mix. Hexadecimal values may not have a fractional part. They are generally converted to decimal in the output.

### QUOTED

Quotes begin with an ASCII single quote `'` or double quote `"` and end with the same character they started with. An ASCII backslash `\` begins an escape sequence; this allows newlines `\n`, tabs `\t`, and quotes `\'`, `\"` to be inserted. It also allows multiline strings by escaping a newline character, which will be ignored.