lib/promrelabel: optimize common regex mismatch cases for action: replace and action: labelmap

lib/promrelabel/config.go

@@ -218,6 +218,7 @@ func parseRelabelConfig(rc *RelabelConfig) (*parsedRelabelConfig, error) {
 		regexOrig := regex
 		if rc.Action != "replace_all" && rc.Action != "labelmap_all" {
 			regex = regexutil.RemoveStartEndAnchors(regex)
+			regexOrig = regex
 			regex = "^(?:" + regex + ")$"
 		}
 		re, err := regexp.Compile(regex)

lib/promrelabel/relabel.go

@@ -203,6 +203,11 @@ func (prc *parsedRelabelConfig) apply(labels []prompbmarshal.Label, labelsOffset
 				return labels
 			}
 		}
+		if !prc.regex.MatchString(bytesutil.ToUnsafeString(bb.B)) {
+			// Fast path - regexp mismatch.
+			relabelBufPool.Put(bb)
+			return labels
+		}
 		match := prc.RegexAnchored.FindSubmatchIndex(bb.B)
 		if match == nil {
 			// Fast path - nothing to replace.
@@ -388,6 +393,10 @@ func (prc *parsedRelabelConfig) replaceFullString(s, replacement string, hasCapt
 			}
 		}
 	}
+	if !prc.regex.MatchString(s) {
+		// Fast path - regex mismatch
+		return s, false
+	}
 	// Slow path - regexp processing
 	match := prc.RegexAnchored.FindStringSubmatchIndex(s)
 	if match == nil {

lib/promrelabel/relabel_timing_test.go

@@ -497,7 +497,7 @@ func BenchmarkApplyRelabelConfigs(b *testing.B) {
 		pcs := mustParseRelabelConfigs(`
 - action: drop
   source_labels: [id]
-  regex: yes
+  regex: "yes"
 `)
 		labelsOrig := []prompbmarshal.Label{
 			{
@@ -584,7 +584,7 @@ func BenchmarkApplyRelabelConfigs(b *testing.B) {
 		pcs := mustParseRelabelConfigs(`
 - action: keep
   source_labels: [id]
-  regex: yes
+  regex: "yes"
 `)
 		labelsOrig := []prompbmarshal.Label{
 			{

lib/regexutil/promregex_test.go

@@ -87,4 +87,6 @@ func TestPromRegex(t *testing.T) {
 	f(".*(a|b).*", "xa", true)
 	f(".*(a|b).*", "xay", true)
 	f(".*(a|b).*", "xzy", false)
+	f("^(?:true)$", "true", true)
+	f("^(?:true)$", "false", false)
 }