def _expressions_from_rules(self, rule_syntax):
"""Return the rules for parsing the grammar definition syntax.
Return a 2-tuple: a dict of rule names pointing to their expressions,
and then the top-level expression for the first rule.
"""
# Hard-code enough of the rules to parse the grammar that describes the
# grammar description language, to bootstrap:
ws = Regex(r'\s+', name='ws')
_ = Regex(r'[ \t]+', name='_')
label = Regex(r'[a-zA-Z_][a-zA-Z_0-9]*', name='label')
quantifier = Regex(r'[*+?]', name='quantifier')
# This pattern supports empty literals. TODO: A problem?
literal = Regex(r'u?r?"[^"\\]*(?:\\.[^"\\]*)*"',
ignore_case=True,
dot_all=True,
name='literal')
regex = Sequence(Literal('~'),
literal,
Regex('[ilmsux]*', ignore_case=True),
name='regex')
atom = OneOf(label, literal, regex, name='atom')
quantified = Sequence(atom, quantifier, name='quantified')
term = OneOf(quantified, atom, name='term')
another_term = Sequence(_, term, name='another_term')
sequence = Sequence(term, OneOrMore(another_term), name='sequence')
or_term = Sequence(_, Literal('/'), another_term, name='or_term')
ored = Sequence(term, OneOrMore(or_term), name='ored')
and_term = Sequence(_, Literal('&'), another_term, name='and_term')
anded = Sequence(term, OneOrMore(and_term), name='anded')
poly_term = OneOf(anded, ored, sequence, name='poly_term')
rhs = OneOf(poly_term, term, name='rhs')
eol = Regex(r'[\r\n$]', name='eol') # TODO: Support $.
rule = Sequence(Optional(ws),
label,
Optional(_),
Literal('='),
Optional(_),
rhs,
Optional(_),
eol,
name='rule')
rules = Sequence(OneOrMore(rule), Optional(ws), name='rules')
# Use those hard-coded rules to parse the (possibly more extensive)
# rule syntax. (For example, unless I start using parentheses in the
# rule language definition itself, I should never have to hard-code
# expressions for those above.)
rule_tree = rules.parse(rule_syntax)
# Turn the parse tree into a map of expressions:
return RuleVisitor().visit(rule_tree)
def test_optional(self):
"""``Optional`` should return its own node wrapping the succeeded child."""
expr = Optional(Literal('a', name='lit'), name='opt')
text = 'a'
eq_(expr.match(text),
Node('opt', text, 0, 1, children=[Node('lit', text, 0, 1)]))
# Test failure of the Literal inside the Optional; the
# LengthTests.test_optional is ambiguous for that.
text = ''
eq_(expr.match(text), Node('opt', text, 0, 0))
def test_optional(self):
len_eq(Sequence(Optional(Literal('a')), Literal('b')).match('b'),
1) # contained expr fails
len_eq(Sequence(Optional(Literal('a')), Literal('b')).match('ab'),
2) # contained expr succeeds