LANG-1307: Added getDigits method to StringUtils (closes #225)

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

@@ -7156,6 +7156,42 @@ public class StringUtils {
         return true;
     }
 
+    /**
+     * <p>Checks if a String {@code str} contains Unicode digits,
+     * if yes then concatenate all the digits in {@code str} and return it as a String.</p>
+     *
+     * <p>An empty ("") String will be returned if no digits found in {@code str}.</p>
+     *
+     * <pre>
+     * StringUtils.getDigits(null)  = null
+     * StringUtils.getDigits("")    = ""
+     * StringUtils.getDigits("abc") = ""
+     * StringUtils.getDigits("1000$") = "1000"
+     * StringUtils.getDigits("1123~45") = "12345"
+     * StringUtils.getDigits("(541) 754-3010") = "5417543010"
+     * StringUtils.getDigits("\u0967\u0968\u0969") = "\u0967\u0968\u0969"
+     * </pre>
+     *
+     * @param str the String to extract digits from, may be null
+     * @return String with only digits,
+     *           or an empty ("") String if no digits found,
+     *           or {@code null} String if {@code str} is null
+     */
+    public static String getDigits(final String str) {
+        if (isEmpty(str)) {
+            return str;
+        }
+        final int sz = str.length();
+        final StringBuilder strDigits = new StringBuilder(sz);
+        for (int i = 0; i < sz; i++) {
+            final char tempChar = str.charAt(i);
+            if (Character.isDigit(tempChar)) {
+                strDigits.append(tempChar);
+            }
+        }
+        return strDigits.toString();
+    }
+
     /**
      * <p>Checks if the CharSequence contains only whitespace.</p>
      * 

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

@@ -3206,4 +3206,15 @@ public class StringUtilsTest {
         assertNull(StringUtils.toCodePoints(null));
         assertArrayEquals(ArrayUtils.EMPTY_INT_ARRAY, StringUtils.toCodePoints(""));
     }
+
+    @Test
+    public void testGetDigits() {
+        assertEquals(null, StringUtils.getDigits(null));
+        assertEquals("", StringUtils.getDigits(""));
+        assertEquals("", StringUtils.getDigits("abc"));
+        assertEquals("1000", StringUtils.getDigits("1000$"));
+        assertEquals("12345", StringUtils.getDigits("123password45"));
+        assertEquals("5417543010", StringUtils.getDigits("(541) 754-3010"));
+        assertEquals("\u0967\u0968\u0969", StringUtils.getDigits("\u0967\u0968\u0969"));
+    }
 }