Fix named parameters normalization issue

Author: YomesInc

cloudinary-core/src/main/java/com/cloudinary/transformation/BaseExpression.java

@@ -123,7 +123,7 @@ private static Pattern getPattern() {
             sb.append(Pattern.quote(op)).append("|");
         }
         sb.deleteCharAt(sb.length() - 1);
-        sb.append(")(?=[ _])|(?<!\\$)(").append(StringUtils.join(PREDEFINED_VARS.keySet(), "|")).append("))");
+        sb.append(")(?=[ _])|(?<![\\$:])(").append(StringUtils.join(PREDEFINED_VARS.keySet(), "|")).append("))");
         pattern = sb.toString();
         return Pattern.compile(pattern);
     }

cloudinary-core/src/test/java/com/cloudinary/transformation/ExpressionTest.java

@@ -174,4 +174,16 @@ public void normalize_doesntReplaceVariable_8() {
         String actual = Expression.normalize("$__height_100");
         assertEquals("$_height_100", actual);
     }
+
+    @Test
+    public void normalize_duration() {
+        String actual = Expression.normalize("duration");
+        assertEquals("du", actual);
+    }
+
+    @Test
+    public void normalize_previewDuration() {
+        String actual = Expression.normalize("preview:duration_2");
+        assertEquals("preview:duration_2", actual);
+    }
 }