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Platform notes

The five ports share one API but reach ICU through different doors, so their feature sets differ slightly at the edges. This page is the quick "what can I use where" reference; for the exhaustive matrix, see Feature parity.

The short version

Reaches ICU via Completeness
Java ICU4J (the reference ICU implementation) most complete — blocked on nothing
C# ICU4C (native ICU C library, P/Invoke, bundled ICU 72) near-Java — four C-API surface gaps
Python PyICU (ICU C++ bindings) near-complete — three small binding quirks
PHP ext-intl (+ raw ResourceBundle, RBNF) v3 reaches everything except likely-subtags & the matcher/index family
JavaScript the standard Intl API everything Intl exposes; raw-ICU features omitted by design

Everything in the Guide without a callout works in all five ports. The exceptions are below.

JavaScript can't do (not in Intl)

Intl is a curated formatting subset of ICU — it has no parser, transform engine, spoof checker, locale matcher, or resource access. Those raw-ICU features are intentionally omitted, not faked. All are available in PHP, Python, Java, and C# (unless noted):

  • spellout() — spell a number out ("one hundred twenty") — needs RBNF
  • ordinal() — ordinal text ("1st", "2nd") (the ordinal plural category is available everywhere via pluralCategory())
  • quote() — wrap text in the locale's quotation marks (CLDR delimiter data)
  • get() — raw ICU ResourceBundle access (PHP, Python & Java only)
  • formatMoment() — arbitrary ICU date/time patterns
  • transliterate() / romanize() — script conversion & ASCII slugs (needs Transliterator)
  • confusable() / suspicious() — spoof detection (needs SpoofChecker)
  • parseNumber() / parseMoney() / parseDate() / parseMoment() — Intl is format-only
  • bestMatch() / negotiating fromAcceptLanguage() — needs LocaleMatcher (Python & Java only)
  • indexBuckets() — needs AlphabeticIndex (Python & Java only)

JavaScript also does not infer a currency from the region: money() requires an explicit currency code or the currency modifier (PHP, Python, Java, and C# infer it).

PHP can't do (no ext-intl API)

PHP v3 closed most of its old gaps by reconstructing the missing formatters from CLDR ResourceBundle data (no hardcoded tables) — compact(), relativeDuration(), numberRange()/moneyRange(), and dateRange() now all work. Two caveats:

  • dateRange() supports short/medium only (CLDR ships no long/full interval skeletons that ext-intl can reach); it throws on long/full.
  • moneyRange() is approximate — it fills the CLDR range pattern but doesn't collapse the shared currency symbol the way native ICU does.

Genuinely still blocked in PHP (no ext-intl API, not cleanly reconstructable — all available in Python and Java):

  • addLikelySubtags() / removeLikelySubtags() — no likely-subtags algorithm
  • bestMatch() / negotiating fromAcceptLanguage() — no LocaleMatcher binding
  • indexBuckets() — no AlphabeticIndex binding

Three Python quirks

Python is near-complete, but PyICU 2.16's bindings leave three small gaps (documented in the Python README):

  • week_info() omits the weekend days. PyICU doesn't bind ICU's getDayOfWeekType / isWeekend, and the no-hardcoded-data rule forbids a fallback table — so it returns first_day + minimal_days only. (PHP, Java, and C# return the weekend too.)
  • relative_duration(numeric="auto") returns the numeric form ("1 day ago", not "yesterday") — PyICU doesn't cleanly expose ICU's "auto" word-forms. The result is always correct, just not colloquial. (PHP, Java, and C# produce the words.)
  • supported_values() covers timeZone/collation/numberingSystem/unit/transliterator but not currency/calendar.

Four C# gaps (ICU4C C-API limits)

C# wraps ICU4C via P/Invoke with bundled ICU 72 native binaries. The four gaps are C-API surface limits, not missing ICU data:

  • indexBuckets() — AlphabeticIndex has no C API (C++ only).
  • bestMatch() / negotiating fromAcceptLanguage() — LocaleMatcher has no C API. The non-negotiating FromAcceptLanguage(header) (no supported list) works.
  • personName() — PersonNameFormatter was added in ICU 73 C++; the bundled ICU is version 72.
  • supportedValues("unit") — uloc_getAvailable doesn't enumerate CLDR measure units; throws CosmoUnsupportedException.

Everything else — RBNF (spellout/ordinal), CLDR delimiters (quote), transliteration, spoof detection, locale-aware parsing, likely-subtags, formatMoment, dateRange (all widths), relative-duration word forms, weekend days in weekInfo, and most supportedValues keys — works fully in C#.

Java's one exclusive — and what it doesn't block

Java (ICU4J) is blocked on nothing: weekend days, auto word forms, and the full supportedValues set (plus transliterator) all work. It also carries the one method no other port can reach:

  • personName() — locale-correct person-name formatting (surname-first locales, initials, formality variants) via ICU 73+'s PersonNameFormatter. JS has only a TC39 proposal, ext-intl doesn't bind it, PyICU 2.16 doesn't expose it, and C# bundles ICU 72 which predates the formatter. See Negotiation & names.

Why omit instead of polyfill?

Every port follows the same rule: if its runtime's ICU can't produce a result, the feature is left out — never reimplemented from a hardcoded table. (PHP's v3 CLDR reconstructions are the one nuance — they still read live ICU ResourceBundle data, so nothing is hardcoded.) That's what keeps all five ports correct and maintenance-free as CLDR/ICU data evolves, and it's why the gaps above are by-design boundaries rather than bugs or backlog.