diff --git a/exporter.go b/exporter.go
index c225fcf..a2ad52f 100644
--- a/exporter.go
+++ b/exporter.go
@@ -275,12 +275,16 @@ type Exporter struct {
 	labelValues map[string]map[uint64]*LabelValues
 }
 
+// Replace invalid characters in the metric name with "_"
+// Valid characters are a-z, A-Z, 0-9, and _
 func escapeMetricName(metricName string) string {
 	// If a metric starts with a digit, prepend an underscore.
 	if len(metricName) > 0 && metricName[0] >= '0' && metricName[0] <= '9' {
 		metricName = "_" + metricName
 	}
 
+	// this is an character replacement method optimized for this limited
+	// use case.  It is much faster than using a regex.
 	out := make([]byte, len(metricName))
 	j := 0
 	for _, c := range metricName {