fix(logql): properly count regexp captures

internal/logql/parser_pipeline.go

@@ -292,10 +292,20 @@ func (p *parser) parseRegexpLabelParser() (*RegexpLabelParser, error) {
 			Err: err,
 		}
 	}
+	if re.NumSubexp() == 0 {
+		return nil, &ParseError{
+			Pos: patternTok.Pos,
+			Err: errors.New("at least one capture expected"),
+		}
+	}
+
+	var (
+		captures = re.SubexpNames()
+		mapping  = make(map[int]Label, len(captures))
+		unique   = make(map[string]struct{}, len(captures))
+	)
 
-	mapping := map[int]Label{}
-	unique := map[string]struct{}{}
-	for i, name := range re.SubexpNames() {
+	for i, name := range captures {
 		// Not capturing.
 		if name == "" {
 			continue
@@ -317,6 +327,12 @@ func (p *parser) parseRegexpLabelParser() (*RegexpLabelParser, error) {
 		}
 		mapping[i] = Label(name)
 	}
+	if len(mapping) == 0 {
+		return nil, &ParseError{
+			Pos: patternTok.Pos,
+			Err: errors.New("at least one capture expected"),
+		}
+	}
 
 	return &RegexpLabelParser{
 		Regexp:  re,

internal/logql/parser_test.go

@@ -1186,6 +1186,9 @@ var tests = []TestCase{
 	{`{} | regexp "\\"`, nil, true},
 	{`{} | foo=~"\\"`, nil, true},
 	{`label_replace(rate({job="mysql"}[1m]), "dst", "replacement", "src", "\\")`, nil, true},
+	// No capture.
+	{`{} | regexp "a"`, nil, true},
+	{`{} | regexp "a (\\w+)"`, nil, true},
 	// Duplicate capture.
 	{`{} | regexp "(?P<method>\\w+)(?P<method>\\w+)"`, nil, true},
 	// Invalid capture name.
@@ -1195,6 +1198,7 @@ var tests = []TestCase{
 	//
 	// No capture.
 	{`{} | pattern "a"`, nil, true},
+	{`{} | pattern "a <_> a"`, nil, true},
 	// Duplicate capture.
 	{`{} | pattern "<a> foo <a>"`, nil, true},
 	// Consecutive capture.