ABNF

ABNF is a package that generates parsers from ABNF grammars as described in RFC 5234 and RFC7405. The main purpose of this package is to parse data as specified in RFCs. But it should be able to handle any ABNF grammar.

ABNF was originally written a few years ago for parsing HTTP headers in a web framework. The code herein has been in use in production here and there on the internet since then.

Requirements

ABNF is tested with Python 3.10-14.

Installation

The abnf package is available from PyPI. As of version 2.3.1, abnf uses trusted publishing.

Install it in the usual way.

pip install abnf

Usage

The main class of abnf is Rule. You should think of a Rule subclass as corresponding to an ABNF grammar. Then instances of that subclass represent the rules of that grammar.

The Rule class is initialized with the core ABNF rules OCTET, BIT, HEXDIG, CTL, HTAB, LWSP, CR, VCHAR, DIGIT, WSP, DQUOTE, LF, SP, CRLF, CHAR, ALPHA, and so are available in any subclass of Rule.

Create a Rule object using the class method create.

rule = Rule.create('double-quoted-string = DQUOTE *(%x20-21 / %x23-7E / %x22.22) DQUOTE')

To later retrieve the object just created:

rule = Rule('double-quoted-string')

Rule objects are cached, so Rule('double-quoted-string') should always return the same object, though you might not want to depend on that.

ABNF includes several grammars. The Rule subclass ABNFGrammarRule implements the rules for ABNF. The package abnf.grammars includes grammars from several RFCs.

from abnf.grammars import rfc7232
src = 'W/"moof"'
node, start = rfc7232.Rule('ETag').parse(src)
print(str(node))

The output is

Node(
    name=ETag, 
    children=
        [
        Node(
            name=entity-tag, 
            children=
                [
                Node(
                    name=weak, 
                    children=
                        [
                        Node(
                            name=literal, 
                            offset=0, 
                            value="W/"
                            )
                        ]
                    ), 
                    Node(
                        name=opaque-tag, 
                        children=
                            [
                            Node(
                                name=DQUOTE, 
                                children=
                                    [
                                    Node(
                                        name=literal, 
                                        offset=2, 
                                        value="""
                                        )
                                    ]
                                ), 
                            Node(
                                name=etagc, 
                                children=
                                    [
                                    Node(
                                        name=literal, 
                                        offset=3, 
                                        value="m"
                                        )
                                    ]
                                ), 
                            Node(
                                name=etagc, 
                                children=
                                    [
                                    Node(
                                        name=literal, 
                                        offset=4, 
                                        value="o"
                                        )
                                    ]
                                ), 
                            Node(
                                name=etagc, 
                                children=
                                    [
                                    Node(
                                        name=literal, 
                                        offset=5, 
                                        value="o"
                                        )
                                    ]
                                ), 
                            Node(
                                name=etagc, 
                                children=
                                [
                                Node(
                                    name=literal, 
                                    offset=6, 
                                    value="f"
                                    )
                                ]
                            ), 
                        Node(
                            name=DQUOTE, 
                            children=
                                [
                                Node(
                                    name=literal, 
                                    offset=7, 
                                    value="""
                                    )
                                ]
                            )
                        ]
                    )
                ]
            )
        ]
    )'

The modules in abnf.grammars may serve as an example for writing other Rule subclasses. In particular, some of the RFC grammars incorporate rules by reference from other RFC. abnf.grammars.rfc7230 shows a way to import rules from another Rule subclass.

You can also load a grammar from a text file using Rule.from_file. This class function accepts either a str or pathlib.Path. The text file must contain an ABNF rulelist.

class FromFileRule(Rule):
    pass

FromFileRule.from_file('/path/to/grammar.abnf')

ABNF uses CRLF as a delimiter for rules in a rulelist. Beware that many text editors (e.g. BBEdit) substitute line endings without telling the user.

Errors

abnf implements two exception subclasses, ParseError and GrammarError.

A GrammarError is raised when parsing encounters an undefined rule, or a prose-value in a grammar.

A ParseError is raised when parsing fails for some reason. Error reporting is nothing more than a stack trace, but that usually allows one to get to the source of the problem.

Examples

Validate an email address

The code below validates an arbitrary email address. If src is not syntactically valid, a ParseError is raised.

from abnf.grammars import rfc5322

src = 'test@example.com'
parser = rfc5322.Rule('address')
parser.parse_all(src)

Extract the actual address from an email address

from abnf.grammars import rfc5322

def get_address(node):
    """Do a breadth-first search of the tree for addr-spec node.  If found, 
    return its value."""
    queue = [node]
    while queue:
        n, queue = queue[0], queue[1:]
        if n.name == 'addr-spec':
            return n.value
        else:
            queue.extend(n.children)
    return None

src = 'John Doe <jdoe@example.com>'
parser = rfc5322.Rule('address')
node = parser.parse_all(src)
address = get_address(node)


for x in node_iterator(node):
    if x.name == 'addr-spec':
        print(x.value)
        break

Extract authentication information from an HTTP Authorization header.

from abnf.parser import NodeVisitor
from abnf.grammars import rfc7235

header_value = 'Basic YWxhZGRpbjpvcGVuc2VzYW1l'
parser = rfc7235.Rule('Authorization')
node, offset = parser.parse(header_value, 0)

class AuthVisitor(NodeVisitor):
    def __init__(self):
        super().__init__()
        self.auth_scheme = None
        self.token = None

    def visit_authorization(self, node):
        for child_node in node.children:
            self.visit(child_node)

    def visit_credentials(self, node):
        for child_node in node.children:
            self.visit(child_node)

    def visit_auth_scheme(self, node):
        self.auth_scheme = node.value

    def visit_token68(self, node):
        self.token = node.value

visitor = AuthVisitor()
visitor.visit(node)

The result is that visitor.auth_scheme = 'Basic', and visitor.token = 'YWxhZGRpbjpvcGVuc2VzYW1l'

Implementation

abnf is implemented using parser combinators. There is a class Literal whose instances are initialized with either a string like 'moof', or a tuple like ('a', 'z') representing a range. The result is a parser that can match the initialized value.

ABNF operations -- alternation, concatenation, repeat, etc. are implemented as classes.
For example, Alternation(Literal('foo'), Literal('bar')) returns a parser that implements the ABNF expression

"foo" / "bar"

The whole mess is bootstrapped by writing out the parsers for the grammar and core rules by hand. The ABNFGrammarRule class represents the ABNF grammar, and is used to parse other grammars. It is also capable of parsing its own grammar.

Alternation

RFC 5234 does not specify the precise behavior of alternation. The ABNF definition of ABNF appears to assume longest match. But other grammars expect first match alternation (e.g. dhall). So this behavior is configurable. A class attribute Rule.firstmatchalternation allows one to choose a behavior for a particular grammar (as represented by a Rule subclass). When firstmatchalternation is False, alternation returns the longest match; in the event of a tie, the first match is returned. When firstmatchalternation is True, the first match is returned.

Backtracking

ABNF implements backtracking as of version 2.0.0. There were sufficient changes in behavior that this constituted a breaking change, and so the major version has been bumped.

As is well-known, naive implementations of backtracking typically have exponential worst-case behavior. Here I attempt to reduce that through the use of generators and some caching. In particular, Repetition objects cache parse results.

Version 2.0.0 uses a LRU cache, ParseCache. The code comes wihout any max sizes set for caches, which will obviously result in long-term issues.
My hope is to get feedback from parser usage. ParseCache has a class attribute maxcachesize: int | None that if set to a non-negative integer, will limit cache size.

Development, Testing, etc.

Should you wish to tinker with the code, install in a virtual environment and have at it. The requirements for a dev environment are specifed in pyproject.toml. To install using pip:

pip install .[dev]

A good starting point would be to run pytest and see that all tests pass.

pytest --cov-report term-missing --cov=abnf

The test suite includes fuzz testing with test data generated using abnfgen. Some of the test rules are long and gruesome. Thus the tests take a bit of time to complete. Skip the fuzz tests with

    pytest --cov-report term-missing --cov=abnf --ignore=tests/fuzz

Following changes, run

pylint abnf

to resolve any problems found, then

tox

to execute tests for python 3.10-3.14.

The code is formatted using black.

black src/abnf

Third-Party Packages

abnf-to-regexp

The program abnf-to-regexp converts augmented Backus-Naur form (ABNF) to a regular expression.

Filters