config: error on missing regex in relabel config.

Fixes issue #787.

config/config.go

@@ -409,7 +409,13 @@ type RelabelConfig struct {
 func (c *RelabelConfig) UnmarshalYAML(unmarshal func(interface{}) error) error {
 	*c = DefaultRelabelConfig
 	type plain RelabelConfig
-	return unmarshal((*plain)(c))
+	if err := unmarshal((*plain)(c)); err != nil {
+		return err
+	}
+	if c.Regex == nil {
+		return fmt.Errorf("relabel configuration requires a regular expression")
+	}
+	return nil
 }
 
 // Regexp encapsulates a regexp.Regexp and makes it YAML marshallable.

config/config_test.go

@@ -164,6 +164,9 @@ var expectedErrors = []struct {
 	}, {
 		filename: "regex.bad.yml",
 		errMsg:   "error parsing regexp",
+	}, {
+		filename: "regex_missing.bad.yml",
+		errMsg:   "relabel configuration requires a regular expression",
 	}, {
 		filename: "rules.bad.yml",
 		errMsg:   "invalid rule file path",

config/testdata/regex_missing.bad.yml

@@ -0,0 +1,4 @@
+scrape_configs:
+  - job_name: prometheus
+    relabel_configs:
+      - source_labels: ['blub']