Canonical peg Grammar

The peg grammar provides the canonical PEG grammar as defined in “Parsing Expression Grammars: A Recognition Based Syntactic Foundation”. It provides a concise rule declaration, freeform layout and literal escape sequences.

Top-level rules

spacing: " "* | end_line | "#" :: end_line

Any amount of whitespace and line-comments is allowed between elements of the grammar:

name  <-  # rule name
    head  # first element
    then  # second element

define: name <- ["/"] rule ("/" rule)*

A named collection of ordered rules. If more than one rule matches, the leftmost matching rule is preferred.

Definitions and rules have no end delimiter; a sequence of definitions is valid:

# any sequence of a's and b's
ab <- a / b a <- "a" ab? b <- "b" ab?

rule: e spacing [ '{' action '}' ] spacing

A rule to match any input matching the expression e. The optional action defines how to translate the matched input.

Compound Expressions

sequence: e1 e2

Ordered sequence of clauses, matching e1 followed by e2.

"return" expression

choice: e1 / e2

Ordered choice, matching either e1 or e2. If both match, e1 is preferred.

group: ( e )

Match e. Useful to enforce precedence.

expr (',' expr)*

option: e?

Match e or nothing. Always succeeds, may be zero width.

"async"? "def"

not: !e

Match if e does not match. Matches with zero width.

EndOfFile <- !.

and: &e

Match if e does match, but with zero width. This is an optimised form of !!e.

begin (& ':') colon_block

repeat: e+

Match e once or several times.

':' statement+

any: e*

Match e zero or several times.

capture: name=e

Capture the result of matching e with a given name for use in a rule action.

Special Terminals

nothing: '' or ""

Zero-length literal, always matches at any position.

anything: .

Match any input of width one.

Literal Terminals

Literals interpret the escape sequences \n, \r, \t, octal escapes of the form \ooo and \oo, and 4-width and 8-with unicode escapes of the form \uhhhh and \Uhhhhhhhh. Use \', \", \[, \], \\ for literal special characters. All other escape sequences are rejected.

literal: " :: " or ' :: '

Match any input exactly equal to the literal.

"def"

range: [start "-" stop] or [first second]

Match any single character in the range. Multiple ranges can be combined:

atoz <- [ab-yzABC-Z]