The grammar consists of a sequence of rules of the form:

rule_name: expression

Optionally, a type can be included right after the rule name, which specifies the return type of the Python function corresponding to the rule:

rule_name[return_type]: expression

If the return type is omitted, then the return type is Any.

Grammar Expressions

# comment

Python-style comments.

e1 e2

Match e1, then match e2.

rule_name: first_rule second_rule

e1 | e2

Match e1 or e2.

The first alternative can also appear on the line after the rule name for formatting purposes. In that case, a | can also be used before the first alternative, like so:

    | first_alt
    | second_alt

( e )

Match e.

rule_name: (e)

A slightly more complex and useful example includes using the grouping operator together with the repeat operators:

rule_name: (e1 e2)*

[ e ] or e?

Optionally match e.

rule_name: [e]

A more useful example includes defining that a trailing comma is optional:

rule_name: e (',' e)* [',']


Match zero or more occurrences of e.

rule_name: (e1 e2)*


Match one or more occurrences of e.

rule_name: (e1 e2)+


Match one or more occurrences of e, separated by s. The generated parse tree does not include the separator. This is otherwise identical to (e (s e)*).

rule_name: ','.e+


Succeed if e can be parsed, without consuming any input.


Fail if e can be parsed, without consuming any input.

An example taken from data/python.gram specifies that a primary consists of an atom, which is not followed by a . or a ( or a [:

primary: atom !'.' !'(' !'['


Commit to the current alternative, even if it fails to parse.

rule_name: '(' ~ some_rule ')' | some_alt

In this example, if a left parenthesis is parsed, then the other alternative won’t be considered, even if some_rule or ‘)’ fail to be parsed.


Fail immediatly if e fails to parse by raising the exception built using the make_syntax_error method.

This construct can help provide better error messages.


Keywords are identified in the grammar as quoted names. Single quotes 'def' are used to identify hard keywords i.e. keywords that are reserved in the grammar and cannot be used for any other purpose. Double quotes "match" identify soft keywords that act as keyword only in specific context. As a consequence a rule matching NAME may match a soft keyword but never a hard keyword.

In some circumstances, it can desirable to match any soft keyword. For those cases one can use SOFT_KEYWORD that will expand to "match" | "case" if match and case are the only two known soft keywords.

Return Value

Optionally, an alternative can be followed by a so-called action in curly-braces, which specifies the return value of the alternative:

    | first_alt1 first_alt2 { first_alt1 }
    | second_alt1 second_alt2 { second_alt1 }

If the action is omitted, a list with all the parsed expressions gets returned. However if the rule contains a single element it is returned as is without being wrapped in a list. Rules allowing to match multiple items (+ or *) always return a list.

By default the parser does not track line number and col offset for production each rule. If one desires to store the start line and offset and the end line and offset of a rule, one can add LOCATIONS in the action. It will be replaced in the generated parser by the value of the location_formatting argument of the parser generator, which defaults to:

lineno=start_lineno, col_offset=start_col_offset, end_lineno=end_lineno, end_col_offset=end_col_offset

The default is suitable to generate Python AST nodes.

Variables in the Grammar

A subexpression can be named by preceding it with an identifier and an = sign. The name can then be used in the action, like this:

rule_name[return_type]: '(' a=some_other_rule ')' { a }

Grammar actions

To avoid the intermediate steps that obscure the relationship between the grammar and the AST generation the PEG parser allows directly generating AST nodes for a rule via grammar actions. Grammar actions are language-specific expressions that are evaluated when a grammar rule is successfully parsed. These expressions can be written in Python. As an example of a grammar with Python actions, the piece of the parser generator that parses grammar files is bootstrapped from a meta-grammar file with Python actions that generate the grammar tree as a result of the parsing.

In the specific case of the PEG grammar for Python, having actions allows directly describing how the AST is composed in the grammar itself, making it more clear and maintainable. This AST generation process is supported by the use of some helper functions that factor out common AST object manipulations and some other required operations that are not directly related to the grammar.

To indicate these actions, each alternative can be followed by an action inside curly-braces, which specifies a Python expression to be evaluated and returned for the alternative:

    | first_alt1 first_alt2 { first_alt1 }
    | second_alt1 second_alt2 { second_alt1 }


The code inside curly-braces can only be a Python expression (i.e. it can be assigned to a variable).

If the action is omitted, a default action is generated:

  • If there’s a single name in the rule in the rule, it gets returned.

  • If there is more than one name in the rule, a collection with all parsed expressions gets returned.

This default behaviour is primarily made for very simple situations and for debugging purposes.

As an illustrative example this simple grammar file allows directly generating a full parser that can parse simple arithmetic expressions and that returns a valid Python AST:

start[ast.Module]: a=expr_stmt* ENDMARKER { ast.Module(body=a or [] }
expr_stmt: a=expr NEWLINE { ast.Expr(value=a, EXTRA) }

    | l=expr '+' r=term { ast.BinOp(left=l, op=ast.Add(), right=r, EXTRA) }
    | l=expr '-' r=term { ast.BinOp(left=l, op=ast.Sub(), right=r, EXTRA) }
    | term

    | l=term '*' r=factor { ast.BinOp(left=l, op=ast.Mult(), right=r, EXTRA) }
    | l=term '/' r=factor { ast.BinOp(left=l, op=ast.Div(), right=r, EXTRA) }
    | factor

    | '(' e=expr ')' { e }
    | atom

    | NAME
    | NUMBER

Left recursion

PEG parsers normally do not support left recursion but Pegen implements a technique that allows left recursion using the memoization cache. This allows us to write not only simple left-recursive rules but also more complicated rules that involve indirect left-recursion like

rule1: rule2 | 'a'
rule2: rule3 | 'b'
rule3: rule1 | 'c'

and “hidden left-recursion” like:

rule: 'optional'? rule '@' some_other_rule

Customizing the generated parser

By default, the generated parser inherits from the Parser class defined in pegen/, and is named GeneratedParser. One can customize the generated module by modifying the header and trailer of the module generated by pegen. To do so one can add dedicated sections to the grammar, which are discussed below:

@class NAME

This allows to specify the name of the generated parser.


Specify a subheader for the module as a string (one can typically use triple quoted string). This is empty by default and is the safer to edit to perform custom imports.


Specify a trailer for the module which is appended to the parser definition. It defaults to MODULE_SUFFIX which is defined in pegen.python_generator. Note that the trailer is formatted using .format(class_name=cls_name) allowing you to reference the created parser in the trailer.

The following snippets illustrates naming the parser MyParser and making the parser inherit from a custom base class.

@class MyParser

@subheader '''
from my_package import BaseParser as Parser


This is not a hard limit, but lines longer than 110 characters should almost always be wrapped. Most lines should be wrapped after the opening action curly brace, like:

really_long_rule[expr_ty]: some_arbitrary_rule {
    _This_is_the_action }