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# This module tries to implement ISO 14977 standard with pyparsing.
# pyparsing version 1.1 or greater is required.
from typing import Any
# ISO 14977 standardize The Extended Backus-Naur Form(EBNF) syntax.
# You can read a final draft version here:
# https://www.cl.cam.ac.uk/~mgk25/iso-ebnf.html
#
# Submitted 2004 by Seo Sanghyeon
# Updated to current pyparsing styles 2025 by Paul McGuire
#
import pyparsing as pp
all_names = """
integer
meta_identifier
terminal_string
optional_sequence
repeated_sequence
grouped_sequence
syntactic_primary
syntactic_factor
syntactic_term
single_definition
definitions_list
syntax_rule
syntax
""".split()
LBRACK, RBRACK, LBRACE, RBRACE, LPAR, RPAR, DASH, STAR, EQ, SEMI = pp.Suppress.using_each(
"[]{}()-*=;"
)
integer = pp.common.integer()
meta_identifier = pp.common.identifier()
terminal_string = pp.Regex(
r'"[^"]*"'
r"|"
r"'[^']*'"
).add_parse_action(pp.remove_quotes)
definitions_list = pp.Forward()
optional_sequence = LBRACK + definitions_list + RBRACK
repeated_sequence = LBRACE + definitions_list + RBRACE
grouped_sequence = LPAR + definitions_list + RPAR
syntactic_primary = (
optional_sequence
| repeated_sequence
| grouped_sequence
| meta_identifier
| terminal_string
)
syntactic_factor = pp.Optional(integer + STAR) + syntactic_primary
syntactic_term = syntactic_factor + pp.Optional(DASH + syntactic_factor)
single_definition = pp.DelimitedList(syntactic_term, ",")
definitions_list <<= pp.DelimitedList(single_definition, "|")
syntax_rule = meta_identifier + EQ + definitions_list + SEMI
ebnfComment = (
("(*" + (pp.CharsNotIn("*") | ("*" + ~pp.Literal(")")))[...] + "*)")
.streamline()
.setName("ebnfComment")
)
syntax = syntax_rule[1, ...]
syntax.ignore(ebnfComment)
def do_integer(toks):
return int(toks[0])
def do_meta_identifier(toks):
if toks[0] in symbol_table:
return symbol_table[toks[0]]
else:
symbol_table[toks[0]] = pp.Forward()
return symbol_table[toks[0]]
def do_terminal_string(toks):
return pp.Literal(toks[0])
def do_optional_sequence(toks):
return pp.Optional(toks[0])
def do_repeated_sequence(toks):
return pp.ZeroOrMore(toks[0])
def do_grouped_sequence(toks):
return pp.Group(toks[0])
def do_syntactic_primary(toks):
return toks[0]
def do_syntactic_factor(toks):
if len(toks) == 2 and toks[0] > 1:
# integer * syntactic_primary
return pp.And([toks[1]] * toks[0])
else:
# syntactic_primary
return [toks[0]]
def do_syntactic_term(toks):
if len(toks) == 2:
# syntactic_factor - syntactic_factor
return pp.NotAny(toks[1]) + toks[0]
else:
# syntactic_factor
return [toks[0]]
def do_single_definition(toks):
toks = toks.asList()
if len(toks) > 1:
# syntactic_term , syntactic_term , ...
return pp.And(toks)
else:
# syntactic_term
return [toks[0]]
def do_definitions_list(toks):
toks = toks.asList()
if len(toks) > 1:
# single_definition | single_definition | ...
return pp.Or(toks)
else:
# single_definition
return [toks[0]]
def do_syntax_rule(toks):
# meta_identifier = definitions_list ;
assert toks[0].expr is None, "Duplicate definition"
toks[0] <<= toks[1]
return [toks[0]]
def do_syntax():
# syntax_rule syntax_rule ...
return symbol_table
for name in all_names:
expr = vars()[name]
action = vars()["do_" + name]
expr.set_name(name)
expr.add_parse_action(action)
# expr.setDebug()
symbol_table: dict[str, pp.Forward] = {}
def parse(ebnf, given_table=None, *, enable_debug=False):
given_table = given_table or {}
symbol_table.clear()
symbol_table.update(given_table)
table = syntax.parse_string(ebnf, parse_all=True)[0]
missing_definitions = [
k for k, v in table.items()
if k not in given_table and v.expr is None
]
assert not missing_definitions, f"Missing definitions for {missing_definitions}"
for name, expr in table.items():
expr.set_name(name)
expr.set_debug(enable_debug)
return table
if __name__ == '__main__':
try:
syntax.create_diagram("ebnf_diagram.html")
except Exception as e:
print("Failed to create diagram for EBNF syntax parser"
f" - {type(e).__name__}: {e}")