fix StringUtils.getDigits dropping supplementary digits

src/main/java/org/apache/commons/lang3/StringUtils.java

@@ -2076,11 +2076,12 @@ public static String getDigits(final String str) {
         final char[] buffer = new char[len];
         int count = 0;
 
-        for (int i = 0; i < len; i++) {
-            final char tempChar = str.charAt(i);
-            if (Character.isDigit(tempChar)) {
-                buffer[count++] = tempChar;
+        for (int i = 0; i < len;) {
+            final int codePoint = str.codePointAt(i);
+            if (Character.isDigit(codePoint)) {
+                count += Character.toChars(codePoint, buffer, count);
             }
+            i += Character.charCount(codePoint);
         }
         return new String(buffer, 0, count);
     }

src/test/java/org/apache/commons/lang3/StringUtilsTest.java

@@ -718,6 +718,14 @@ void testGetDigitsKeycaps() {
         assertEquals("0123456789", StringUtils.getDigits("0️⃣1️⃣2️⃣3️⃣4️⃣5️⃣6️⃣7️⃣8️⃣9️⃣#️⃣"));
     }
 
+    @Test
+    void testGetDigitsSupplementary() {
+        // U+1D7CF MATHEMATICAL BOLD DIGIT ONE: Character.isDigit is true but it is a surrogate pair
+        final String mathOne = new String(Character.toChars(0x1D7CF));
+        assertEquals(mathOne, StringUtils.getDigits(mathOne));
+        assertEquals(mathOne + "9", StringUtils.getDigits("a" + mathOne + "9"));
+    }
+
     @Test
     void testGetFuzzyDistance() {
         assertEquals(0, StringUtils.getFuzzyDistance("", "", Locale.ENGLISH));