Validate input against a grammar

To check that a whole string conforms to a rule, use parse_all: it parses from the start and raises ParseError unless the entire input is consumed.

from abnf.grammars import rfc5322
from abnf import ParseError

def is_valid_email(src: str) -> bool:
    try:
        rfc5322.Rule("address").parse_all(src)
        return True
    except ParseError:
        return False

is_valid_email("test@example.com")          # True
is_valid_email("not an address")            # False

parse vs. parse_all

• parse(source, start) returns (node, offset) and stops at the longest match it finds — it does not require consuming the whole string. Use it when the rule is a prefix of a larger input, and inspect the returned offset.

• parse_all(source) returns the node and raises ParseError if any input is left over. Use it for validation, when the entire string must match.

node, offset = rfc5322.Rule("address").parse("test@example.com and more", 0)
# offset points just past the address; parse_all would have raised here

Note

A ParseError carries the parser and offset at which parsing failed. A GrammarError (a different exception) means the grammar itself is unusable at that point — an undefined rule or a prose-value — not that the input was invalid.