+ * The substr plugin requires that the source value is not empty. If empty
+ * values are expected, combine skip_on_empty process plugin to the pipeline:
+ * @code
+ * process:
+ * new_text_field:
+ * -
+ * plugin: skip_on_empty
+ * method: process
+ * source: some_text_field
+ * -
+ * plugin: substr
+ * source: some_text_field
+ * start: 6
+ * length: 10
+ * @endcode
+ *