Stop ExtendedMessageFormat seekNonWs reading past the pattern end

src/main/java/org/apache/commons/text/ExtendedMessageFormat.java

@@ -478,12 +478,14 @@ private int readArgumentIndex(final String pattern, final ParsePosition pos) {
      * @param pos current position.
      */
     private void seekNonWs(final String pattern, final ParsePosition pos) {
-        int len = 0;
         final char[] buffer = pattern.toCharArray();
-        do {
-            len = StringMatcherFactory.INSTANCE.splitMatcher().isMatch(buffer, pos.getIndex(), 0, buffer.length);
+        while (pos.getIndex() < buffer.length) {
+            final int len = StringMatcherFactory.INSTANCE.splitMatcher().isMatch(buffer, pos.getIndex(), 0, buffer.length);
+            if (len == 0) {
+                break;
+            }
             pos.setIndex(pos.getIndex() + len);
-        } while (len > 0 && pos.getIndex() < pattern.length());
+        }
     }
 
     /**

src/test/java/org/apache/commons/text/ExtendedMessageFormatTest.java

@@ -236,6 +236,17 @@ void testArgumentIndexTrailingWhitespaceAtEnd() {
         assertThrowsExactly(IllegalArgumentException.class, () -> new ExtendedMessageFormat("{0 ", new HashMap<>()));
     }
 
+    @Test
+    void testTruncatedFormatElementWithRegistry() {
+        // A format element left unterminated at the end of the pattern used to make the
+        // registry-based constructor seek one past the buffer and throw
+        // ArrayIndexOutOfBoundsException; it should report the documented
+        // IllegalArgumentException, as the plain constructor already does.
+        assertThrowsExactly(IllegalArgumentException.class, () -> new ExtendedMessageFormat("{", new HashMap<>()));
+        assertThrowsExactly(IllegalArgumentException.class, () -> new ExtendedMessageFormat("{0,", new HashMap<>()));
+        assertThrowsExactly(IllegalArgumentException.class, () -> new ExtendedMessageFormat("{0}extra{", new HashMap<>()));
+    }
+
     /**
      * Test the built in choice format.
      */