host_normalize: Normalize hosts to canonical comparison form

In punycoder: Unicode and Punycode Domain Name Processing

Normalize hosts to canonical comparison form

Description

Converts DNS hostnames to their canonical comparison form following the ratified canonical-host normalization contract: Unicode NFC, case mapping, UTS-46 label mapping and validation (non-transitional, with UseSTD3ASCIIRules, CheckHyphens, CheckBidi, and CheckJoiners), conversion to lowercase ASCII A-labels, and DNS length verification, while preserving whether the input carried a single terminal root dot.

Usage

host_normalize(
  x,
  check_hyphens = TRUE,
  use_std3 = TRUE,
  verify_dns_length = TRUE
)

Arguments

x	Character vector of hostnames. NA elements pass through as NA (missing, not invalid). Names are preserved.
check_hyphens	Logical scalar. When TRUE (the default) the UTS #46 CheckHyphens rule rejects "--" in the 3rd/4th positions and leading or trailing hyphens. FALSE drops that check.
use_std3	Logical scalar. When TRUE (the default) UseSTD3ASCIIRules restricts ASCII to letters, digits, and hyphen. FALSE admits other ASCII (e.g. "_") that the pinned table marks STD3-disallowed-but-valid.
verify_dns_length	Logical scalar. When TRUE (the default) each A-label must be 1-63 octets and the whole host <= 253. FALSE drops the length limits (empty labels are still rejected as structural errors).

Details

Unlike puny_encode(), invalid input is reported by returning NA_character_ (never by aborting), so a caller can layer its own policy. The profile is fixed at one pinned Unicode version per release; see normalization_profile_info() for the machine-readable identity.

This is a UTS #46 profile, not IDNA2008 / RFC 5891 conformance. UTS #46 is compatibility processing and deliberately differs from IDNA2008 — it accepts labels IDNA2008 would reject (e.g. a label whose first character is the symbol U+2615 HOT BEVERAGE becomes "xn--53h.example"). The pipeline draws on RFC 3492 (the Punycode transform), NFC per UAX #15, the RFC 5892 ContextJ rules via CheckJoiners (ZWJ/ZWNJ only — full RFC 5892 CONTEXTO is not checked), the RFC 5893 Bidi rule via CheckBidi, and STD 3 (RFC 952 + RFC 1123) host-name rules via UseSTD3ASCIIRules. IDNA2003 / Nameprep (RFC 3490/3491/3454) is not used.

The default applies the full strict UTS #46 profile (uts46-nontransitional-std3-v1). The check_hyphens, use_std3, and verify_dns_length arguments are UTS #46 processing flags that can each be relaxed independently; pass the same values to normalization_profile_info() to obtain the identity of the resulting profile. These are standard UTS #46 parameters, not a browser mode: CheckBidi and CheckJoiners always apply and are never knobs, and full WHATWG host policy (where beStrict = false flips exactly these three) lives upstack in rurl, not here.

Value

A character vector the same length as x. Each element is the canonical lowercase ASCII A-label host, or NA_character_ when the input is NA or invalid under the profile.

See Also

normalization_profile_info() for the profile identity, puny_encode() for the lower-level RFC 3492 transform.

Examples

host_normalize(c("Example.COM", "münchen.de", "example.com."))
host_normalize("a_b.com") # NA: STD3 rejects "_"
host_normalize("a_b.com", use_std3 = FALSE) # "a_b.com"

punycoder documentation built on July 20, 2026, 1:07 a.m.