Put unescaped regexes in a raw string literal (#10331)

[This PR](https://github.com/grafana/loki/pull/10277) fixed an issue
where regexes were being escaped multiple times and causing otherwise
valid queries to fail our length check. It put the resulting regexes in
double quotes which meant that any regexes that _needed_ escaping were
now invalid.

This PR make is so the unescaped regex is put in a raw string literal so
any extra escaping in unnecessary.

pkg/logql/log/label_filter.go

@@ -368,6 +368,16 @@ type lineFilterLabelFilter struct {
 	filter Filterer
 }
 
+// overrides the matcher.String() function in case there is a regexpFilter
+func (s *lineFilterLabelFilter) String() string {
+	if unwrappedFilter, ok := s.filter.(regexpFilter); ok {
+		rStr := unwrappedFilter.String()
+		str := fmt.Sprintf("%s%s`%s`", s.Matcher.Name, s.Matcher.Type, rStr)
+		return str
+	}
+	return s.Matcher.String()
+}
+
 func (s *lineFilterLabelFilter) Process(_ int64, line []byte, lbs *LabelsBuilder) ([]byte, bool) {
 	v := labelValue(s.Name, lbs)
 	return line, s.filter.Filter(unsafeGetBytes(v))

pkg/logql/syntax/parser_test.go

@@ -3465,3 +3465,22 @@ func TestNoOpLabelToString(t *testing.T) {
 	require.NoError(t, err)
 	require.Len(t, stages, 0)
 }
+
+func TestParseSampleExpr_String(t *testing.T) {
+	t.Run("it doesn't add escape characters when after getting parsed", func(t *testing.T) {
+		query := `sum(rate({cluster="beep", namespace="boop"} | msg=~` + "`" + `.*?(GET|POST|PUT|DELETE|PATCH|HEAD|OPTIONS) /loki/api/(?i)(\d+[a-z]|[a-z]+\d)\w*/query_range` + "`" + `[1d]))`
+		expr, err := ParseSampleExpr(query)
+		require.NoError(t, err)
+
+		require.Equal(t, query, expr.String())
+	})
+
+	t.Run("it removes escape characters after being parsed", func(t *testing.T) {
+		query := `{cluster="beep", namespace="boop"} | msg=~"\\w.*"`
+		expr, err := ParseExpr(query)
+		require.NoError(t, err)
+
+		// escaping is hard: the result is {cluster="beep", namespace="boop"} | msg=~`\w.*` which is equivalent to the original
+		require.Equal(t, "{cluster=\"beep\", namespace=\"boop\"} | msg=~`\\w.*`", expr.String())
+	})
+}