Fix lang() to match whole language subtags, not any prefix

src/main/java/org/apache/commons/jxpath/ri/model/NodePointer.java

@@ -750,12 +750,27 @@ protected boolean isDefaultNamespace(final String prefix) {
      * Check whether our locale matches the specified language.
      *
      * @param lang String language to check
-     * @return true if the selected locale name starts with the specified prefix <em>lang</em>, case-insensitive.
+     * @return true if the selected locale name matches <em>lang</em> under the XPath {@code lang()} rules, case-insensitive.
      */
     public boolean isLanguage(final String lang) {
         final Locale loc = getLocale();
         final String name = loc.toString().replace('_', '-');
-        return name.toUpperCase(Locale.ENGLISH).startsWith(lang.toUpperCase(Locale.ENGLISH));
+        return isLanguage(name, lang);
+    }
+
+    /**
+     * Tests whether the language tag {@code value} matches {@code lang} under the XPath 1.0 {@code lang()} rules: the comparison is case-insensitive and
+     * {@code lang} must equal the whole tag or a leading subtag delimited by {@code '-'}. So {@code "en"} matches {@code "en"} and {@code "en-US"} but not
+     * {@code "english"}, and the bare prefix {@code "e"} matches neither.
+     *
+     * @param value the language tag to test, for example an {@code xml:lang} value or a locale name
+     * @param lang  the language being tested for
+     * @return whether {@code value} is {@code lang} or a sublanguage of it
+     */
+    protected static boolean isLanguage(final String value, final String lang) {
+        final String name = value.toUpperCase(Locale.ENGLISH);
+        final String target = lang.toUpperCase(Locale.ENGLISH);
+        return name.equals(target) || name.startsWith(target + "-");
     }
 
     /**

src/main/java/org/apache/commons/jxpath/ri/model/dom/DOMNodePointer.java

@@ -679,7 +679,7 @@ public boolean isCollection() {
     @Override
     public boolean isLanguage(final String lang) {
         final String current = getLanguage();
-        return current == null ? super.isLanguage(lang) : current.toUpperCase(Locale.ENGLISH).startsWith(lang.toUpperCase(Locale.ENGLISH));
+        return current == null ? super.isLanguage(lang) : isLanguage(current, lang);
     }
 
     @Override

src/main/java/org/apache/commons/jxpath/ri/model/jdom/JDOMNodePointer.java

@@ -696,7 +696,7 @@ public boolean isCollection() {
     @Override
     public boolean isLanguage(final String lang) {
         final String current = getLanguage();
-        return current == null ? super.isLanguage(lang) : current.toUpperCase(Locale.ENGLISH).startsWith(lang.toUpperCase(Locale.ENGLISH));
+        return current == null ? super.isLanguage(lang) : isLanguage(current, lang);
     }
 
     @Override

src/test/java/org/apache/commons/jxpath/ri/model/AbstractBeanModelTest.java

@@ -78,7 +78,10 @@ void testAttributeLang() {
         assertXPathValue(context, "@xml:lang", "en-US");
         assertXPathValue(context, "count(@xml:*)", Double.valueOf(1));
         assertXPathValue(context, "lang('en')", Boolean.TRUE);
+        assertXPathValue(context, "lang('en-US')", Boolean.TRUE);
         assertXPathValue(context, "lang('fr')", Boolean.FALSE);
+        // lang() must match a whole subtag, not any leading prefix
+        assertXPathValue(context, "lang('e')", Boolean.FALSE);
     }
 
     /**

src/test/java/org/apache/commons/jxpath/ri/model/AbstractXMLModelTest.java

@@ -360,6 +360,8 @@ void testLang() {
         assertXPathValue(context, "//product/prix/@xml:lang", "fr");
         // lang() used the built-in xml:lang attribute
         assertXPathValue(context, "//product/prix[lang('fr')]", "934.99");
+        // a leading prefix that is not a whole subtag must not match xml:lang="fr"
+        assertXPathValue(context, "count(//product/prix[lang('f')])", Double.valueOf(0));
         // Default language
         assertXPathValue(context, "//product/price:sale[lang('en')]/saleEnds", "never");
     }