Merge pull request #447 from pedro-stanaka/feat/doc-special-regex-match

Adding documentation about special regex match group

README.md

@@ -317,6 +317,27 @@ mappings:
     provider: "$1"
 ```
 
+#### Special match groups
+
+When using regex, the match group `0` is the full match and can be used to attach labels to the metric.
+Example:
+
+```yaml
+mappings:
+- match: ".+"
+  match_type: regex
+  name: "$0"
+  labels:
+    statsd_metric_name: "$0"
+```
+
+If a metric `my.statsd_counter` is received, the metric name will **still** be mapped to `my_statsd_counter` (Prometheus compatible name).
+But the metric will also have the label `statsd_metric_name` with the value `my.statsd_counter` (unchanged value).
+
+Note: If you use the `match` like the example (i.e. `.+`), be aware that it will be a "catch-all" block. So it should come at the very end of the mapping list.
+
+The same is not achievable with glob matching, for more details check [this issue](https://github.com/prometheus/statsd_exporter/issues/444).
+
 ### Naming, labels, and help
 
 Please note that metrics with the same name must also have the same set of