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Grako (for "grammar compiler") takes a grammar in a variation of EBNF as input, and outputs a memoizing PEG/Packrat parser in Python.

Project description

At least for the people who send me mail about a new language that they’re designing, the general advice is: do it to learn about how to write a compiler. Don’t have any expectations that anyone will use it, unless you hook up with some sort of organization in a position to push it hard. It’s a lottery, and some can buy a lot of the tickets. There are plenty of beautiful languages (more beautiful than C) that didn’t catch on. But someone does win the lottery, and doing a language at least teaches you something.

Dennis Ritchie (1941-2011) Creator of the C programming language and of Unix


Grako (for grammar compiler) is a tool that takes grammars in a variation of EBNF as input, and outputs memoizing (Packrat) PEG parsers in Python.

Grako is different from other PEG parser generators:

  • Generated parsers use Python’s very efficient exception-handling system to backtrack. Grako generated parsers simply assert what must be parsed. There are no complicated if-then-else sequences for decision making or backtracking. Memoization allows going over the same input sequence several times in linear time.

  • Positive and negative lookaheads, and the cut element (with its cleaning of the memoization cache) allow for additional, hand-crafted optimizations at the grammar level.

  • Delegation to Python’s re module for lexemes allows for (Perl-like) powerful and efficient lexical analysis.

  • The use of Python’s context managers considerably reduces the size of the generated parsers for code clarity, and enhanced CPU-cache hits.

  • Include files, rule inheritance, and rule inclusion give Grako grammars considerable expressive power.

  • Efficient support for direct and indirect left recursion allows for more intuitive grammars.

The parser-generator, the runtime support, and the generated parsers have measurably low Cyclomatic complexity. At around 4 KLOC of Python, it is possible to study all its source code in a single session.

The only dependencies are on the Python 2.7, 3.4, or PyPy 2.3 standard libraries (the proposed regex module will be used if installed, colorama will be used on trace output if available, and pygraphviz is required for generating diagrams). For performance beyond what which Python or PyPy can provide, take a look at the Grako++ project.

Grako is feature-complete and currently being used with complex grammars to parse and translate hundreds of thousands of lines of legacy code in programming languages like NATURAL, COBOL, VB6, and Java.

Table of Contents


Grako was created to address recurring problems encountered over decades of working with parser generation tools:

  • Many languages allow the use of certain keywords as identifiers, or have different meanings for symbols depending on context (Ruby). A parser needs to be able to control the lexical analysis to handle those languages.

  • LL and LR grammars become contaminated with myriads of lookahead statements to deal with ambiguous constructs in the source language. PEG parsers address ambiguity from the onset.

  • Separating the grammar from the code that implements the semantics, and using a variation of a well-known grammar syntax (EBNF in this case), allows for full declarative power in language descriptions. General-purpose programming languages are not up to the task.

  • Semantic actions do not belong in a grammar. They create yet another programming language to deal with when doing parsing and translation: the source language, the grammar language, the semantics language, the generated parser’s language, and the translation’s target language. Most grammar parsers do not check that the embedded semantic actions have correct syntax, so errors get reported at awkward moments, and against the generated code, not against the source.

  • Preprocessing (like dealing with includes, fixed column formats, or structure-through-indentation) belongs in well-designed program code; not in the grammar.

  • It is easy to recruit help with knowledge about a mainstream programming language (Python in this case), but it’s hard for grammar-description languages. Grako grammars are in the spirit of a Translators and Interpreters 101 course (if something is hard to explain to a college student, it’s probably too complicated, or not well understood).

  • Generated parsers should be easy to read and debug by humans. Looking at the generated source code is sometimes the only way to find problems in a grammar, the semantic actions, or in the parser generator itself. It’s inconvenient to trust generated code that you cannot understand.

  • Python is a great language for working with language parsing and translation.

The Generated Parsers

A Grako generated parser consists of the following classes:

  • A parser class derived from Parser which implements the parser using one method for each grammar rule:

    def _myrulename_(self):
  • A semantics delegate class with one semantic method per grammar rule. Each method receives as its single parameter the Abstract Syntax Tree (AST) built from the rule invocation:

    def myrulename(self, ast):
        return ast

The methods in the delegate class return the same AST received as parameter, but custom semantic classes can override the methods to have them return anything (for example, a Semantic Graph). The semantics class can be used as a template for the final semantics implementation, which can omit methods for the rules it is not interested in.

If present, a _default() method will be called in the semantics class when no method matched the rule name:

def _default(self, ast):
    return ast

If present, a _postproc() method will be called in the semantics class after each rule (including the semantics) is processed. This method will receive the current parsing context as parameter:

def _postproc(self, context, ast):

Using the Tool

Grako can be run from the command line:

$ python -m grako


$ scripts/grako

Or just:

$ grako

if Grako was installed using easy_install or pip.

The -h and –help parameters provide full usage information:

$ python -m grako -h
usage: grako [-h] [-b] [-d] [-n] [-m NAME] [-o FILE] [-p] [-t] [-v] [-w CHARACTERS]

GRAKO (for "grammar compiler") takes grammars in a variation of EBNF as input,
and outputs a memoizing PEG/Packrat parser in Python.

positional arguments:
GRAMMAR               The filename of the Grako grammar

optional arguments:
-h, --help            show this help message and exit
-b, --binary          generate a pickled grammar model (requires --output)
-c, --color           use color in traces (requires the colorama library)
-d, --draw            generate a diagram of the grammar (requires --output)
-l, --no-left-recursion
                      turns left-recusion support off
-n, --no-nameguard    allow tokens that are prefixes of others
-m NAME, --name NAME  Name for the grammar (defaults to GRAMMAR base name)
-o FILE, --output FILE
                        output file (default is stdout)
-p, --pretty          prettify the input grammar
-t, --trace           produce verbose parsing output
-v, --version         provide version information and exit
                        characters to skip during parsing (use "" to disable)

Using the Generated Parser

To use the generated parser, just subclass the base or the abstract parser, create an instance of it, and invoke its parse() method passing the grammar to parse and the starting rule’s name as parameter:

parser = MyParser()
ast = parser.parse('text to parse', rule_name='start')
print(json.dumps(ast, indent=2)) # ASTs are JSON-friendy

This is more or less what happens if you invoke the generated parser directly:

python inputfile startrule

The generated parsers’ constructors accept named arguments to specify whitespace characters, the regular expression for comments, case sensitivity, verbosity, and more (see below).

To add semantic actions, just pass a semantic delegate to the parse method:

model = parser.parse(text, rule_name='start', semantics=MySemantics())

If special lexical treatment is required (like in Python’s structure-through-indentation), then a descendant of grako.buffering.Buffer can be passed instead of the text:

class MySpecialBuffer(grako.buffering.Buffer):

buf = MySpecialBuffer(text)
model = parser.parse(buf, rule_name='start', semantics=MySemantics())

The EBNF Grammar Syntax

Grako uses a variant of the standard EBNF syntax. Syntax definitions for VIM can be found under the etc/vim directory in the source code distribution.


A grammar consists of a sequence of one or more rules of the form:

name = <expre> ;

If a name collides with a Python keyword, an underscore (_) will be appended to it on the generated parser.

Rule names that start with an uppercase character:

FRAGMENT = /[a-z]+/ ;

do not advance over whitespace before beginning to parse. This feature becomes handy when defining complex lexical elements, as it allows breaking them into several rules.


The expressions, in reverse order of operator precedence, can be:

e1 | e2

Match either e1 or e2.

e1 e2

Match e1 and then match e2.

( e )

Grouping. Match e. For example: ('a' | 'b').

[ e ]

Optionally match e.

{ e } or { e }*

Closure. Match e zero or more times. Note that the AST returned for a closure is always a list.

{ e }+ or { e }-

Positive closure. Match e one or more times. The AST is always a list.


Empty closure. Match nothing and produce an empty list as AST.

s.{ e }

Inspired by Python’s str.join(), is equivalent to:

e {s ~ e}

The s part is not included in the resulting AST.

Use grouping if s is more complex than a token or a pattern:

(s t).{ e }

Use an optional if empty sequences are allowed:

[ s.{ e } ]

To return an empty list as AST if there is no sequence:

( s.{e}|{} )

Positive lookahead. Try parsing e, but do not consume any input.


Negative lookahead. Try parsing e and fail if there’s a match. Do not consume any input whichever the outcome.


The include operator. Include the right hand side of rule rulename at this point.

The following set of declarations:

includable = exp1 ;

expanded = exp0 >includable exp2 ;

Has the same effect as defining expanded as:

expanded = exp0 exp1 exp2 ;

Note that the included rule must be defined before the rule that includes it.

'text' or "text"

Match the token text within the quotation marks.

Note that if text is alphanumeric, then Grako will check that the character following the token is not alphanumeric. This is done to prevent tokens like IN matching when the text ahead is INITIALIZE. This feature can be turned off by passing nameguard=False to the Parser or the Buffer, or by using a pattern expression (see below) instead of a token expression. Alternatively, the @@nameguard directive may be specified in the grammar:

@@nameguard :: False

The pattern expression. Match the Python regular expression regexp at the current text position. Unlike other expressions, this one does not advance over whitespace or comments. For that, place the regexp as the only term in its own rule.

The regexp is passed as-is to the Python re module (or regex if available), using match() at the current position in the text. The matched text is the AST for the expression.


Another form of the pattern expression that can be used when there are slashes (/) in the pattern.


Match nothing, but behave as if constant had been parsed.

Constants can be used to inject elements into the concrete and abstract syntax trees, perhaps avoiding having to write a semantic action. For example:

boolean_option = name ['=' boolean|`true`] ;

Invoke the rule named rulename. To help with lexical aspects of grammars, rules with names that begin with an uppercase letter will not advance the input over whitespace or comments.


The empty expression. Succeed without advancing over input. Its value is None.


The fail expression. This is actually ! applied to (), which always fails.


The cut expression. After this point, prevent other options from being considered even if the current option fails to parse.


Another form of the cut operator. Deprecated.


Add the result of e to the AST using name as key. If name collides with any attribute or method of dict, or is a Python keyword, an underscore (_) will be appended to the name.


Add the result of e to the AST using name as key. Force the entry to be a list even if only one element is added. Collisions with dict attributes or Python keywords are resolved by appending an underscore to name.


The override operator. Make the AST for the complete rule be the AST for e.

The override operator is useful to recover only part of the right hand side of a rule without the need to name it, or add a semantic action.

This is a typical use of the override operator:

subexp = '(' @:expre ')' ;

The AST returned for the subexp rule will be the AST recovered from invoking expre.


Another form of the override operator. Deprecated.


Like @:e, but make the AST always be a list.

This operator is convenient in cases such as:

arglist = '(' @+:arg {',' @+:arg}* ')' ;

In which the delimiting tokens are of no interest.


The end of text symbol. Verify that the end of the input text has been reached.

(* comment *)

Comments may appear anywhere in the text.

# comment

Python-style comments are also allowed.

When there are no named items in a rule, the AST consists of the elements parsed by the rule, either a single item or a list. This default behavior makes it easier to write simple rules:

number = /[0-9]+/ ;

Without having to write:

number = number:/[0-9]+/ ;

When a rule has named elements, the unnamed ones are excluded from the AST (they are ignored).

Rules with Arguments

Grako allows rules to specify Python-style arguments:

addition(Add, op='+')
    addend '+' addend

The arguments values are fixed at grammar-compilation time.

An alternative syntax is available if no keyword parameters are required:

addition::Add, '+'
    addend '+' addend

Semantic methods must be ready to receive any arguments declared in the corresponding rule:

def addition(self, ast, name, op=None):

When working with rule arguments, it is good to define a _default() method that is ready to take any combination of standard and keyword arguments:

def _default(self, ast, *args, **kwargs):

Based Rules

Rules may extend previously defined rules using the < operator. The base rule must be defined previously in the grammar.

The following set of declarations:

base::Param = exp1 ;

extended < base = exp2 ;

Has the same effect as defining extended as:

extended::Param = exp1 exp2 ;

Parameters from the base rule are copied to the new rule if the new rule doesn’t define its own. Repeated inheritance should be possible, but it hasn’t been tested.

Rule Overrides

A grammar rule may be redefined by using the @override decorator:

start = ab $;

ab = 'xyz' ;

ab = @:'a' {@:'b'} ;

When combined with the #include directive, rule overrides can be used to create a modified grammar without altering the original.

Abstract Syntax Trees (ASTs)

By default, and AST is either a list (for closures and rules without named elements), or dict-derived object that contains one item for every named element in the grammar rule. Items can be accessed through the standard dict syntax (ast['key']), or as attributes (ast.key).

AST entries are single values if only one item was associated with a name, or lists if more than one item was matched. There’s a provision in the grammar syntax (the +: operator) to force an AST entry to be a list even if only one element was matched. The value for named elements that were not found during the parse (perhaps because they are optional) is None.

When the parseinfo=True keyword argument has been passed to the Parser constructor, a parseinfo element is added to AST nodes that are dict-like. The element contains a collections.namedtuple with the parse information for the node:

ParseInfo = namedtuple('ParseInfo', ['buffer', 'rule', 'pos', 'endpos'])

With the help of the Buffer.line_info() method, it is possible to recover the line, column, and original text parsed for the node. Note that when ParseInfo is generated, the Buffer used during parsing is kept in memory for the lifetime of the AST.


By default, Grako generated parsers skip the usual whitespace characters with the regular expression r'\s+' using the re.UNICODE flag (or with the Pattern_White_Space property if the regex module is available), but you can change that behavior by passing a whitespace parameter to your parser.

For example, the following will skip over tab (\t) and space characters, but not so with other typical whitespace characters such as newline (\n):

parser = MyParser(text, whitespace='\t ')

The character string is converted into a regular expression character set before starting to parse.

You can also provide a regular expression directly instead of a string. The following is equivalent to the above example:

parser = MyParser(text, whitespace=re.compile(r'[\t ]+'))

Note that the regular expression must be pre-compiled to let Grako distinguish it from plain string.

If you do not define any whitespace characters, then you will have to handle whitespace in your grammar rules (as it’s often done in PEG parsers):

parser = MyParser(text, whitespace='')

Whitespace may also be specified within the grammar using the @@whitespace directive, although any of the above methods will overwrite the grammar directive:

@@whitespace :: /[\t ]+/

Case Sensitivity

If the source language is case insensitive, you can tell your parser by using the ignorecase parameter:

parser = MyParser(text, ignorecase=True)

You may also specify case insensitivity within the grammar using the @@ignorecase directive:

@@ignorecase :: True

The change will affect both token and pattern matching.


Parsers will skip over comments specified as a regular expression using the comments_re parameter:

parser = MyParser(text, comments_re="\(\*.*?\*\)")

For more complex comment handling, you can override the Buffer.eat_comments() method.

For flexibility, it is possible to specify a pattern for end-of-line comments separately:

parser = MyParser(

Both patterns may also be specified within a grammar using the @@comments and @@eol_comments directives:

@@comments :: /\(\*.*?\*\)/
@@eol_comments :: /#.*?$/

Left Recursion

Grako provides support for left recursion in PEG grammars.

Sometimes, while debugging a grammar, it is useful to turn left-recursion support off:

parser = MyParser(

Left recursion can also be turned off from within the grammar using the @@left_recursion directive:

@@left_recursion :: False

Semantic Actions

There are no constructs for semantic actions in Grako grammars. This is on purpose, because semantic actions obscure the declarative nature of grammars and provide for poor modularization from the parser-execution perspective.

Semantic actions are defined in a class, and applied by passing an object of the class to the parse() method of the parser as the semantics= parameter. Grako will invoke the method that matches the name of the grammar rule every time the rule parses. The argument to the method will be the AST constructed from the right-hand-side of the rule:

class MySemantics(object):
    def some_rule_name(self, ast):
        return ''.join(ast)

    def _default(self, ast):

If there’s no method matching the rule’s name, Grako will try to invoke a _default() method if it’s defined:

def _default(self, ast):

Nothing will happen if neither the per-rule method nor _default() are defined.

The per-rule methods in classes implementing the semantics provide enough opportunity to do rule post-processing operations, like verifications (for inadequate use of keywords as identifiers), or AST transformation:

class MyLanguageSemantics(object):
    def identifier(self, ast):
        if my_lange_module.is_keyword(ast):
            raise FailedSemantics('"%s" is a keyword' % str(ast))
        return ast

For finer-grained control it is enough to declare more rules, as the impact on the parsing times will be minimal.

If preprocessing is required at some point, it is enough to place invocations of empty rules where appropriate:

myrule = first_part preproc {second_part} ;

preproc = () ;

The abstract parser will honor as a semantic action a method declared as:

def preproc(self, ast):

Include Directive

Grako grammars support file inclusion through the include directive:

#include :: "filename"

The resolution of the filename is relative to the directory/folder of the source. Absolute paths and ../ navigations are honored.

The functionality required for implementing includes is available to all Grako-generated parsers through the Buffer class; see the GrakoBuffer class in the grako.parser module for an example.

Templates and Translation

Grako doesn’t impose a way to create translators with it, but it exposes the facilities it uses to generate the Python source code for parsers.

Translation in Grako is template-based, but instead of defining or using a complex templating engine (yet another language), it relies on the simple but powerful string.Formatter of the Python standard library. The templates are simple strings that, in Grako’s style, are inlined with the code.

To generate a parser, Grako constructs an object model of the parsed grammar. A grako.codegen.CodeGenerator instance matches model objects to classes that descend from grako.codegen.ModelRenderer and implement the translation and rendering using string templates. Templates are left-trimmed on whitespace, like Python doc-comments are. This is an example taken from Grako’s source code:

class Lookahead(ModelRenderer):
    template = '''\
                with self._if():

Every attribute of the object that doesn’t start with an underscore (_) may be used as a template field, and fields can be added or modified by overriding the render_fields(fields) method. Fields themselves are lazily rendered before being expanded by the template, so a field may be an instance of a ModelRenderer descendant.

The rendering module defines a Formatter enhanced to support the rendering of items in an iterable one by one. The syntax to achieve that is:


All of ind, sep, and fmt are optional, but the three colons are not. A field specified that way will be rendered using:

indent(sep.join(fmt % render(v) for v in value), ind)

The extended format can also be used with non-iterables, in which case the rendering will be:

indent(fmt % render(value), ind)

The default multiplier for ind is 4, but that can be overridden using n*m (for example 3*1) in the format.


Using a newline (\\n) as separator will interfere with left trimming and indentation of templates. To use newline as separator, specify it as \\\\n, and the renderer will understand the intention.



The file etc/grako.ebnf contains a grammar for the Grako EBNF language written in the same Grako grammar language. It is used in the bootstrap test suite to prove that Grako can generate a parser to parse its own language, and the resulting parser is made the bootstrap parser every time Grako is stable (see grako/ for the generated parser). Grako uses Grako to translate grammars into parsers, so it is a good example of end-to-end translation.


The project examples/regexp contains a regexp-to-EBNF translator and parser generator. The project has no practical use, but it’s a complete, end-to-end example of how to implement a translator using Grako.


The project examples/antlr2grako contains a ANTLR to Grako grammar translator. The project is a good example of the use of models and templates in translation. The program, generates the Grako grammar on standard output, but because the model used is Grako’s own, the same code can be used to directly generate a parser from an ANTLR grammar. Please take a look at the examples README to know about limitations.

Other Open-source Examples

  • Christian Ledermann wrote parsewkt a parser for Well-known text (WTK) using Grako.

  • Marcus Brinkmann (lambdafu) wrote, a parser for a MediaWiki-style language.

  • Marcus Brinkmann (lambdafu) is working on a C++ code generator for Grako called Grako++. Help in the form of testing, test cases, and pull requests is welcome.


Grako is Copyright (C) 2012-2014 by Thomas Bragg and Juancarlo Añez

You may use the tool under the terms of the BSD-style license described in the enclosed LICENSE.txt file. If your project requires different licensing please email.

Contact and Updates

For general Q&A, please use the [grako] tag on StackOverflow.

To discuss Grako and to receive notifications about new releases, please join the low-volume Grako Forum at Google Groups.

You can also follow the latest Grako developments with @GrakoPEG on Twitter.


The following must be mentioned as contributors of thoughts, ideas, code, and funding to the Grako project:

  • Niklaus Wirth was the chief designer of the programming languages Euler, Algol W, Pascal, Modula, Modula-2, Oberon, and Oberon-2. In the last chapter of his 1976 book Algorithms + Data Structures = Programs, Wirth creates a top-down, descent parser with recovery for the Pascal-like, LL(1) programming language PL/0. The structure of the program is that of a PEG parser, though the concept of PEG wasn’t formalized until 2004.

  • Bryan Ford introduced PEG (parsing expression grammars) in 2004.

  • Other parser generators like PEG.js by David Majda inspired the work in Grako.

  • William Thompson inspired the use of context managers with his blog post that I knew about through the invaluable Python Weekly newsletter, curated by Rahul Chaudhary

  • Jeff Knupp explains why Grako’s use of exceptions is sound, so I don’t have to.

  • Terence Parr created ANTLR, probably the most solid and professional parser generator out there. Ter, ANTLR, and the folks on the ANLTR forums helped me shape my ideas about Grako.

  • JavaCC (originally Jack) looks like an abandoned project. It was the first parser generator I used while teaching.

  • Grako is very fast. But dealing with millions of lines of legacy source code in a matter of minutes would be impossible without PyPy, the work of Armin Rigo and the PyPy team.

  • Guido van Rossum created and has lead the development of the Python programming environment for over a decade. A tool like Grako, at under six thousand lines of code, would not have been possible without Python.

  • Kota Mizushima welcomed me to the CSAIL at MIT PEG and Packrat parsing mailing list, and immediately offered ideas and pointed me to documentation about the implementation of cut in modern parsers. The optimization of memoization information in Grako is thanks to one of his papers.

  • My students at UCAB inspired me to think about how grammar-based parser generation could be made more approachable.

  • Gustavo Lau was my professor of Language Theory at USB, and he was kind enough to be my tutor in a thesis project on programming languages that was more than I could chew. My peers, and then teaching advisers Alberto Torres, and Enzo Chiariotti formed a team with Gustavo to challenge us with programming languages like LATORTA and term exams that went well into the eight hours. And, of course, there was also the pirate patch that should be worn on the left or right eye depending on the LL or LR challenge.

  • Manuel Rey led me through another, unfinished, thesis project that taught me about what languages (spoken languages in general, and programming languages in particular) are about. I learned why languages use declensions, and why, although the underlying words are in English, the structure of the programs we write is more like Japanese.

  • Marcus Brinkmann has kindly submitted patches that have resolved obscure bugs in Grako’s implementation, and that have made the tool more user-friendly, specially for newcomers to parsing and translation.

  • Robert Speer cleaned up the nonsense in trying to have Unicode handling be compatible with 2.7.x and 3.x, and figured out the canonical way of honoring escape sequences in grammar tokens without throwing off the encoding.

  • Basel Shishani has been an incredibly throrough peer-reviewer of Grako.

  • Paul Sargent implemented Warth et al’s algorithm for supporting direct and indirect left recursion in PEG parsers.

  • Kathryn Long proposed better support for UNICODE in the treatment of whitespace and regular expressions (patterns) in general. Her other contributions have made Grako more congruent, and more user-friendly.

  • David Röthlisberger provided the definitive patch that allows the use of Python keywords as rule names.

  • Grako would not have been possible without the vision, the funding, and the trust provided by Thomas Bragg through ResQSoft.


Grako uses Semantic Versioning for its releases, so parts of the version number may increase without any significant changes or backwards incompatibilities in the software.


  • Added suport for `constant` expressions which don’t consume any input yet return the specified constant.

  • Now an empty closure ({}) consumes no input and generates an empty list as AST.

  • Removed the --binary command-line option. It went unused, it was untested, and it was incorrectly implemented.

  • Generated parsers pass on KeyboardInterrupt.

  • Moved the bulk of the entry code for generated parsers to util.generic_main(). This allows for the verbose code to be verified by the usual tools.

  • Deprecate {e}* and {e}- by removing them from the documentation.

  • Added the Python inspired join operator, s.{e}, as a convenient syntax for parsing sequences with separators.


  • Sever minor BUG fixes. See the commit log for details.


  • BUG Detect and fail promptly on empty tokens in grammars.

  • More reasonable treatment for ANTLR token definitions in the antlr2grako example.


  • All tests pass with Python 3.5.

  • 59 Python keywords can now actually be used as rule names in grammars (drothlis).

  • 60 @@ directives were not pressent in the output of the --pretty option.


  • 58 The parameters to the constructor of generated parsers were being ignored (pgebhard).

  • BUG would call ctx.error() instead of ctx._error() on failed rule references.

  • Overall cleanup of the code and of the development requirements.


  • 56 Using @@whitespace generated invalid python programs

  • The @@whitespace directive was not working for regular expressions (nehz).


  • BUG: Left recursion in the grammar was checked for in the wrong place when disabled.

  • Added basic support for output of an AST in YAML format.


  • Added @@whitespace directive to specify whitespace regular expression within the grammar (starkat).

  • Added @@nameguard and @@ignorecase directives to toggle the respective boolean parameters within the grammar (starkat).

  • 52 Build with Cython failed on Windows.

  • Applied Flake8 suggestions.

  • Upgraded development libraries to their latest versions (see requirements.txt).


  • 45 The grako tool now produces basic statistics about the processed grammar.

  • 46 Left recursion support can be turned off using the left_recursion= parameter to parser constructors.

  • 47 New @@comments and @@eol_comments can be used within a grammar to specify the respective regular expressions.

  • 48 Rules can now be overriden/redefined using the @override decorator.

  • Added backwards compatibility with Buffer.whitespace.

  • Added AST.asjson() to not have to import grako.util.asjson() for the same purpose.


  • Minor improvements to buffering.Buffer.

  • BUG 42 might give errors under some locales because of the non-ASCII characters in README.rst.

  • Added a --no-nameguard command-line option to generated parsers.

  • Allow Buffer descendants to customize how text is split into lines (starkat).

  • Now the re.UNICODE flag is consistently used in pattern, comment, and whitespace matching. A re regular expression is now accepted for whitespace matching. Character sets provided as str, list, or set are converted to the corresponding regular expression (starkat).

  • If installed, the regex module will be used instead of re in all pattern matching (starkat). See the section about whitespace above.

  • Added a --version option to the commandline tool. A grako.__version__ variable is now available.


  • Refactorings to enhance consistency in parsing between models and and generated parsers.

  • 37 Block comments are preserved when using the --pretty option.

  • 38 Trace output uses color if the colorama package is installed. Also, the vertical size of trace logs was reduced to three lines per entry.

  • 40 The widht and the separator used in parse traces are now configurable with keyword arguments.


  • Now rule parameters and model.ModelBuilderSemantics are used to produce grammar models with a minimal set of semantic methods.

  • Code generation is now separtate from the grammar model, so translation targets differen from Python are easier to implement.

  • Removed attribute assignment to the underlying dict in AST. It was the source of obscure bugs for Grako users.

  • Now an eol_comments_re= parameter can be passed to Parser and Buffer.

  • BUG Need to allow newline (\n) characters within grammar patterns.

  • BUG 36 Keyword arguments in rules were not being parsed correctly (Franz_G).

  • Several BUGs in the advanced features were fixed. See the Bitbucket commits for details.


  • Grako now supports direct and indirect left recursion thanks to the implementation done by Paul Sargent of the work by Warth et al. Performance for non-left-recursive grammars is unaffected.

  • The old grammar syntax is now supported with deprecation warnings. Use the --pretty option to upgrade a grammar.

  • If there are no slashes in a pattern, they can now be specified without the opening and closing question marks.

  • BUG 33 Closures were sometimes being treated as plain lists, and that produced inconsistent results for named elements (lambdafu).

  • BUG The bootstrap parser contained errors due to the previous bug in util.ustr().

  • BUG 30 Make sure that escapes in --whitespace are evaluated before being passed to the model.

  • BUG 30 Make sure that --whitespace and --no-nameguard indeed affect the behavior of the generated parser as expected.


  • The bump in the major version number is because the grammar syntax changed to accomodate new features better, and to remove sources of ambituity and hard-to-find bugs. The naming changes in some of the advanced features (Walker) should impact only complex projects.

  • The cut operator is now ~, the tilde.

  • Now name overrides must always be specified with a colon, @:e.

  • Grammar rules may declare Python-style arguments that get passed to their corresponding semantic methods.

  • Grammar rules may now inherit the contents of other rules using the < operator.

  • The right hand side of a rule may be included in another rule using the > operator.

  • Grammars may include other files using the #include :: directive.

  • Multiple definitions of grammar rules with the same name are now disallowed. They created ambiguity with new features such as rule parameters, based rules, and rule inclusion, and they were an opportunity for hard-to-find bugs (import this).

  • Added a --pretty option to the command-line tool, and refactored pretty-printing (__str__() in grammar models) enough to make its output a norm for grammar format.

  • Internals and examples were upgraded to use the latest Grako features.

  • Parsing exceptions will now show the sequence of rule invocations that led to the failure.

  • Renamed Traverser and traverse to Walker and walk.

  • Now the keys in grako.ast.AST are ordered like in collections.OrderedDict.

  • Grako models are now more JSON-friendly with the help of grako.ast.AST.__json__(), grako.model.Node.__json__() and grako.util.asjon().

  • Added compatibility with Cython.

  • Removed checking for compatibility with Python 3.3 (use 3.4 instead).

  • Incorporated Robert Speer’s solution to honoring escape sequences without messing up the encoding.

  • BUG Honor simple escape sequences in tokens while trying not to corrupt unicode input. Projects using non-ASCII characters in grammars should prefer to use unicode character literals instead of Python \x or \o escape sequences. There is no standard/stable way to unscape a Python string with escaped escape sequences. Unicode is broken in Python 2.x.

  • BUG The --list option was not working in Python 3.4.1.

  • BUG 22 Always exit with non-zero exit code on failure.

  • BUG 23 Incorrect encoding of Python escape sequences in grammar tokens.

  • BUG 24 Incorrect template for –pretty of multi-line optionals.


  • Changes to allow downstream translators to have different target languages with as little code replication as possible. There’s new functionality pulled from downstream in grako.model and grako.rendering. grako.model is now a module instead of a package.

  • The Visitor Pattern doesn’t make much sense in a dynamically typed language, so the functionality was replaced by more flexible Traverser classes. The new _traverse_XX() methods in Traverser classes carry a leading underscore to remind that they shouldn’t be used outside of the protocol.

  • Now a _default() method is called in the semantics delegate when no specific method is found. This allows, for example, generating meaningful errors when something in the semantics is missing.

  • Added compatibility with tox. Now tests are performed against the latest releases of Python 2.7.x and 3.x, and PyPy 2.x.

  • Added --whitespace parameter to generated main().

  • Applied Flake8 to project and to generated parsers.


  • Now the @ operator behaves as a special case of the name: operator, allowing for simplification of the grammar, parser, semantics, and Grako grammars. It also allows for expressions such as @+:e, with the expected semantics.

  • Refactoring The functionality that was almost identical in generated parsers and in models was refactored into Context.

  • BUG! Improve consistency of use Unicode between Python 2.7 and 3.x.

  • BUG! Compatibility between Python 2.7/3.x print() statements.


  • Optionally, do not memoize during positive or negative lookaheads. This allows lookaheads to fail semantically without committing to the fail.

  • Fixed the implementation of the optional operator so the AST/CST generated when the optional succeeds is exactly the same as if the expression had been mandatory.

  • Grouping expressions no longer produce a list as CST.

  • BUG! Again, make sure closures always return a list.

  • Added infrastructure for stateful rules (lambdafu, see the pull request ).

  • Again, protect the names of methods for rules with a leading and trailing underscore. It’s the only way to avoid unexpected name clashes.

  • The bootstrap parser is now the one generated by Grako from the bootstrap grammar.

  • Several minor bug fixes (lambdafu).

  • BUG! The choice operator must restore context even when some of the choices match partially and then fail.

  • BUG! Grammar.parse() needs to initialize the AST stack.

  • BUG! AST.copy() was too shallow, so an AST could be modified by a closure iteration that matched partially and eventually failed. Now AST.copy() clones AST values of type list to avoid that situation.

  • BUG! A failed cut must trickle up the rule-call hierarchy so parsing errors are reported as close to their source as possible.


  • Grako no longer assumes that parsers implement the semantics. A separate semantics implementation must be provided. This allows for less polluted namespaces and smaller classes.

  • A last_node protocol allowed the removal of all mentions of variable _e from generated parsers, which are thus more readable.

  • Refactored closures to be more pythonic (there are no anonymous blocks in Python!).

  • Fixes to the antlr2grako example to let it convert over 6000 lines of an ANTLR grammar to Grako.

  • Improved rendering of grammars by grammar models.

  • Now tokens accept Python escape sequences.

  • Added a simple Visitor Pattern for Renderer nodes. Used it to implement diagramming.

  • Create a basic diagram of a grammar if pygraphviz is available. Added the --draw option to the command-line tool.

  • BUG! Trace information off by one character (thanks to lambdafu).

  • BUG! The AST for a closure might fold repeated symbols (thanks to lambdafu).

  • BUG! It was not possible to pass buffering parameters such as whitespace to the parser’s constructor (thanks to lambdafu).

  • Added command-line and parser options to specify the buffering treatment of whitespace and nameguard (lambdafu).

  • Several improvements and bug fixes (mostly by lambdafu).


  • BUG! Sometimes the AST for a closure ({}) was not a list.

  • Semantic actions can now be implemented by a delegate.

  • Reset synthetic method count and use decorators to increase readability of generated parsers.

  • The Grako EBNF grammar and the bootstrap parser now align, so the grammar can be used to bootstrap Grako.

  • The bootstrap parser was refactored to use semantic delegates.

  • Proved that grammar models can be pickled, unpickled, and reused.

  • Added the antlr example with an ANTLR-to-Grako grammar translator.

  • Changed the licensing to simplified BSD.


  • Important memory optimization! Remove the memoization information that a cut makes obsolete (thanks to Kota Mizushima).

  • Make sure that cut actually applies to the nearest fork.

  • Finish aligning model parsing with generated code parsing.

  • Report all the rules missing in a grammar before aborting.

  • Align the sample etc/grako.ebnf grammar to the language parsed by the bootstrap parser.

  • Ensure compatibility with Python 2.7.4 and 3.3.1.

  • Update credits.


  • Lazy rendering of template fields.

  • Optimization of rendering engine’s indent() and trim().

  • Rendering of iterables using a specified separator, indent, and format.

  • Basic documentation of the rendering engine.

  • Added a cache of compiled regexps to Buffer.

  • Align bootstrap parser with generated parser framework.

  • Add cuts to bootstrap parser so errors are reported closer to their origin.

  • (minor) BUG! FailedCut exceptions must translate to their nested exception so the reported line and column make sense.

  • Prettify the sample Grako grammar.

  • Remove or comment-out code for tagged/named rule names (they don’t work, and their usefulness is doubtful).

  • Spell-check this document with Vim spell.

  • Lint using flake8.


  • BUG! Need to preserve state when closure iterations match partially.

  • Improved performance by also memoizing exception results and advancement over whitespace and comments.

  • Work with Unicode while rendering.

  • Improved consistency between the way generated parsers and models parse.

  • Added a table of contents to this README.

  • Document parseinfo and default it to False.

  • Mention the use of context managers.


  • First public release.

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