Parsing Expression Grammars in Rust

This is a simple parser generator based on the Parsing Expression Grammar formalism.

Usage

As a syntax extension

Add to your Cargo.toml:

[dependencies] peg = "~0.1.0"

Add to your crate root: ```

![feature(phase)]

[phase(plugin)] extern crate pegsyntaxext;

```

Use peg_file! modname("mygrammarfile.rustpeg"); to include the grammar from an external file. The macro expands into a module called modname with functions corresponding to the #[pub] rules in your grammar.

Or, use peg! modname(r#" // grammar rules here "#);`

to embed a short PEG grammar inline in your Rust source file. Example.

As a standalone code generator

Run peg input_file.rustpeg to compile a grammar and generate Rust code on stdout.

Grammar Syntax

```

[pub]

rule_name -> type = expression ```

If a rule is marked with #[pub], the generated module has a public function that begins parsing at that rule.

• . - match any single character
• "literal" - match a literal string
• [a-z] - match a single character from a set
• [^a-z] - match a single character not in a set
• rule - match a production defined elsewhere in the grammar and return its result
• expression* - Match zero or more repetitions of expression and return the results as a Vec
• expression+ - Match one or more repetitions of expression and return the results as a Vec
• expression? - Match one or zero repetitions of expression. Returns an Option
• &expression - Match only if expression matches at this position, without consuming any characters
• !expression - Match only if expression does not match at this position, without consuming any characters
• expression ** delim - Match zero or more repetitions of expression delimited with delim and return the results as a Vec
• expression ++ delim - Match one or more repetitions of expression delimited with delim and return the results as a Vec
• e1 / e2 / e3 - Try to match e1. If the match succeeds, return its result, otherwise try e2, and so on.
• e1 e2 e3 - Match expressions in sequence a:e1 b:e2 c:e3 { rust } - Match e1, e2, e3 in sequence. If they match successfully, run the Rust code in the action and return its result. The variables before the colons in the preceding sequence are bound to the results of the corresponding expressions

Match actions can extract data from the match using these variables:

• match_str - the matched string, as a &str slice. Examples:

name -> String = [a-zA-Z0-9_]+ { match_str.to_string() }

number -> int = [0-9]+ { from_str::<uint>(match_str).unwrap() }

• start_pos - the index into the string at which the match starts, inclusive
• pos - the index into the string at which the match ends, exclusive

To Do

• Improve parse error reporting
• Caching
• Support passing user-specified objects (e.g. filename for source mapping, string interner) into action code