From 8147ed4d9b59eeba12023916ecb57e389190c45d Mon Sep 17 00:00:00 2001
From: Wahome Macharia <wahome@Wahomes-MacBook-Pro.local>
Date: Tue, 21 Feb 2023 19:03:35 +0300
Subject: [PATCH] Implement RFC 5646 compliant regex to validate language tags
 and locales

---
 src/lib/isLocale.js     | 110 ++++++++++++++++++++++++++++++++++++++--
 test/validators.test.js |  35 +++++++++++++
 2 files changed, 140 insertions(+), 5 deletions(-)

diff --git a/src/lib/isLocale.js b/src/lib/isLocale.js
index cacac8aec..ec84c8fce 100644
--- a/src/lib/isLocale.js
+++ b/src/lib/isLocale.js
@@ -1,11 +1,111 @@
 import assertString from './util/assertString';
 
-const localeReg = /^[A-Za-z]{2,4}([_-]([A-Za-z]{4}|[\d]{3}))?([_-]([A-Za-z]{2}|[\d]{3}))?$/;
+/*
+  = 3ALPHA              ; selected ISO 639 codes
+    *2("-" 3ALPHA)      ; permanently reserved
+ */
+const extlang = '([A-Za-z]{3}(-[A-Za-z]{3}){0,2})';
+
+/*
+  = 2*3ALPHA            ; shortest ISO 639 code
+    ["-" extlang]       ; sometimes followed by
+                        ; extended language subtags
+  / 4ALPHA              ; or reserved for future use
+  / 5*8ALPHA            ; or registered language subtag
+ */
+const language = `(([a-zA-Z]{2,3}(-${extlang})?)|([a-zA-Z]{5,8}))`;
+
+/*
+  = 4ALPHA              ; ISO 15924 code
+ */
+const script = '([A-Za-z]{4})';
+
+/*
+  = 2ALPHA              ; ISO 3166-1 code
+  / 3DIGIT              ; UN M.49 code
+ */
+const region = '([A-Za-z]{2}|\\d{3})';
+
+/*
+  = 5*8alphanum         ; registered variants
+  / (DIGIT 3alphanum)
+ */
+const variant = '([A-Za-z0-9]{5,8}|(\\d[A-Z-a-z0-9]{3}))';
+
+/*
+  = DIGIT               ; 0 - 9
+  / %x41-57             ; A - W
+  / %x59-5A             ; Y - Z
+  / %x61-77             ; a - w
+  / %x79-7A             ; y - z
+ */
+const singleton = '(\\d|[A-W]|[Y-Z]|[a-w]|[y-z])';
+
+/*
+  = singleton 1*("-" (2*8alphanum))
+                        ; Single alphanumerics
+                        ; "x" reserved for private use
+ */
+const extension = `(${singleton}(-[A-Za-z0-9]{2,8})+)`;
+
+/*
+  = "x" 1*("-" (1*8alphanum))
+ */
+const privateuse = '(x(-[A-Za-z0-9]{1,8})+)';
+
+// irregular tags do not match the 'langtag' production and would not
+// otherwise be considered 'well-formed'. These tags are all valid, but
+// most are deprecated in favor of more modern subtags or subtag combination
+
+const irregular = '((en-GB-oed)|(i-ami)|(i-bnn)|(i-default)|(i-enochian)|' +
+    '(i-hak)|(i-klingon)|(i-lux)|(i-mingo)|(i-navajo)|(i-pwn)|(i-tao)|' +
+    '(i-tay)|(i-tsu)|(sgn-BE-FR)|(sgn-BE-NL)|(sgn-CH-DE))';
+
+// regular tags match the 'langtag' production, but their subtags are not
+// extended language or variant subtags: their meaning is defined by
+// their registration and all of these are deprecated in favor of a more
+// modern subtag or sequence of subtags
+
+const regular = '((art-lojban)|(cel-gaulish)|(no-bok)|(no-nyn)|(zh-guoyu)|' +
+    '(zh-hakka)|(zh-min)|(zh-min-nan)|(zh-xiang))';
+
+/*
+  = irregular           ; non-redundant tags registered
+  / regular             ; during the RFC 3066 era
+
+ */
+const grandfathered = `(${irregular}|${regular})`;
+
+/*
+  RFC 5646 defines delimitation of subtags via a hyphen:
+
+      "Subtag" refers to a specific section of a tag, delimited by a
+      hyphen, such as the subtags 'zh', 'Hant', and 'CN' in the tag "zh-
+      Hant-CN".  Examples of subtags in this document are enclosed in
+      single quotes ('Hant')
+
+  However, we need to add "_" to maintain the existing behaviour.
+ */
+const delimiter = '(-|_)';
+
+/*
+  = language
+    ["-" script]
+    ["-" region]
+    *("-" variant)
+    *("-" extension)
+    ["-" privateuse]
+ */
+const langtag = `${language}(${delimiter}${script})?(${delimiter}${region})?(${delimiter}${variant})*(${delimiter}${extension})*(${delimiter}${privateuse})?`;
+
+/*
+  Regex implementation based on BCP RFC 5646
+  Tags for Identifying Languages
+  https://www.rfc-editor.org/rfc/rfc5646.html
+ */
+const languageTagRegex = new RegExp(`(^${privateuse}$)|(^${grandfathered}$)|(^${langtag}$)`);
 
 export default function isLocale(str) {
   assertString(str);
-  if (str === 'en_US_POSIX' || str === 'ca_ES_VALENCIA') {
-    return true;
-  }
-  return localeReg.test(str);
+  return languageTagRegex.test(str);
 }
diff --git a/test/validators.test.js b/test/validators.test.js
index 239f172ca..611ce9de8 100644
--- a/test/validators.test.js
+++ b/test/validators.test.js
@@ -4816,16 +4816,51 @@ describe('Validators', () => {
         'uz_Latn_UZ',
         'en',
         'gsw',
+        'en-US',
         'es_ES',
+        'es-419',
         'sw_KE',
         'am_ET',
+        'zh-CHS',
         'ca_ES_VALENCIA',
         'en_US_POSIX',
+        'hak-CN',
+        'zh-Hant',
+        'zh-Hans',
+        'sr-Cyrl',
+        'sr-Latn',
+        'zh-cmn-Hans-CN',
+        'cmn-Hans-CN',
+        'zh-yue-HK',
+        'yue-HK',
+        'zh-Hans-CN',
+        'sr-Latn-RS',
+        'sl-rozaj',
+        'sl-rozaj-biske',
+        'sl-nedis',
+        'de-CH-1901',
+        'sl-IT-nedis',
+        'hy-Latn-IT-arevela',
+        'i-enochian',
+        'en-scotland-fonipa',
+        'sl-IT-rozaj-biske-1994',
+        'de-CH-x-phonebk',
+        'az-Arab-x-AZE-derbend',
+        'x-whatever',
+        'qaa-Qaaa-QM-x-southern',
+        'de-Qaaa',
+        'sr-Latn-QM',
+        'sr-Qaaa-RS',
+        'en-US-u-islamcal',
+        'zh-CN-a-myext-x-private',
+        'en-a-myext-b-another',
       ],
       invalid: [
         'lo_POP',
         '12',
         '12_DD',
+        'de-419-DE',
+        'a-DE',
       ],
     });
   });