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.
ABNF is tested with Python 3.9-13.
The abnf package is available from PyPI.
Install it in the usual way.
pip install abnf
The abnf package is signed with GPG. The public key is available from github,
or OpenPGP. The key fingerprint is 3A27 290F D243 BD83 BC3F 5BC8 86C0 57F9 6A41 A77B.
Once you have imported the public key into GPG, you can check the signature by downloading
the files and the signature files from PyPI. No
download links for the signature files are present; you need to create them by appending .asc
to the package URLs.
Once downloaded, use gpg to verify the signatures.
gpg --verify abnf-1.0.0.tar.gz.asc abnf-1.0.0.tar.gz
gpg --verify abnf-1.0.0-py2.py3-none-any.whl.asc abnf-1.0.0-py2.py3-none-any.whl
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.
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.
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)
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
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'
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.
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.first_match_alternation allows one to choose a behavior for a particular grammar (as represented by a Rule subclass). When first_match_alternation is False, alternation returns the longest match; in the event of a tie, the first match is returned. When first_match_alternation is True, the first match is returned.
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 max_cache_size: int | None that if set to a non-negative integer, will
limit cache size.
Should you wish to tinker with the code, install in a virtual environment and have at it. The file requirements.txt contains the packages I use for testing and such.
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 3.9-3.13.
The code is formatted using black.
black src/abnf
The program abnf-to-regexp converts augmented Backus-Naur form (ABNF) to a regular expression.