feat(isLocale): validate isLocale

- it validates different languages for different geographical location
- Closes validatorjs#954

README.md

@@ -128,6 +128,7 @@ Validator                               | Description
 **isJWT(str)**                         | check if the string is valid JWT token.
 **isLatLong(str)**                      | check if the string is a valid latitude-longitude coordinate in the format `lat,long` or `lat, long`.
 **isLength(str [, options])**              | check if the string's length falls in a range.<br/><br/>`options` is an object which defaults to `{min:0, max: undefined}`. Note: this function takes into account surrogate pairs.
+**isLocale(str)**                       | check if the string is a locale
 **isLowercase(str)**                    | check if the string is lowercase.
 **isMACAddress(str)**                   | check if the string is a MAC address.<br/><br/>`options` is an object which defaults to `{no_colons: false}`. If `no_colons` is true, the validator will allow MAC addresses without the colons. Also, it allows the use of hyphens or spaces e.g  '01 02 03 04 05 ab' or '01-02-03-04-05-ab'.
 **isMagnetURI(str)**                      | check if the string is a [magnet uri format](https://en.wikipedia.org/wiki/Magnet_URI_scheme).

src/index.js

@@ -14,6 +14,7 @@ import isIPRange from './lib/isIPRange';
 import isFQDN from './lib/isFQDN';
 
 import isBoolean from './lib/isBoolean';
+import isLocale from './lib/isLocale';
 
 import isAlpha, { locales as isAlphaLocales } from './lib/isAlpha';
 import isAlphanumeric, { locales as isAlphanumericLocales } from './lib/isAlphanumeric';
@@ -157,6 +158,7 @@ const validator = {
   isJSON,
   isEmpty,
   isLength,
+  isLocale,
   isByteLength,
   isUUID,
   isMongoId,

src/lib/isLocale.js

@@ -0,0 +1,11 @@
+import assertString from './util/assertString';
+
+const localeReg = /^[A-z]{2,4}([_-]([A-z]{4}|[\d]{3}))?([_-]([A-z]{2}|[\d]{3}))?$/;
+
+export default function isLocale(str) {
+  assertString(str);
+  if (str === 'en_US_POSIX' || str === 'ca_ES_VALENCIA') {
+    return true;
+  }
+  return localeReg.test(str);
+}

test/validators.js

@@ -2939,6 +2939,27 @@ describe('Validators', () => {
     });
   });
 
+  it('should validate isLocale codes', () => {
+    test({
+      validator: 'isLocale',
+      valid: [
+        'uz_Latn_UZ',
+        'en',
+        'gsw',
+        'es_ES',
+        'sw_KE',
+        'am_ET',
+        'ca_ES_VALENCIA',
+        'en_US_POSIX',
+      ],
+      invalid: [
+        'lo_POP',
+        '12',
+        '12_DD',
+      ],
+    });
+  });
+
   it('should validate strings by byte length (deprecated api)', () => {
     test({
       validator: 'isByteLength',