from pyparsing import (
    Literal,
    ZeroOrMore,
    Forward,
    stringStart,
    stringEnd,
)

def disable(item, elem):
    """Prevent elem from matching inside of item."""
    return item  # what to do here?

atom = Literal("a")

op1 = Literal("*")
op2 = Literal("+")

no1open = Literal("(")
no1close = Literal(")")

expr = Forward()

expr0 = atom | no1open + disable(expr, op1) + no1close
expr1 = expr0 + ZeroOrMore(op1 + expr0)
expr <<= expr1 + ZeroOrMore(op2 + expr1)

grammar = stringStart + expr + stringEnd

def match_in(text):
    """Determines if there is a match for grammar in text."""
    for result in grammar.parseWithTabs().scanString(text):
        return True
    return False

assert match_in("a * a")
assert match_in("a + a")
assert match_in("a * a + a * a")
assert match_in("a + (a + a)")
assert match_in("a * (a + a)")
assert not match_in("a + (a * a)")  # fails, because disable does nothing