Allow single-letter identifiers (metric and label names).

rules/lexer.l

@@ -45,7 +45,7 @@ AVG|SUM|MAX|MIN          { yylval.str = yytext; return AGGR_OP }
 [*/%]                    { yylval.str = yytext; return MULT_OP }
 
 {D}+{U}                  { yylval.str = yytext; return DURATION }
-{L}({L}|{D})+            { yylval.str = yytext; return IDENTIFIER }
+{L}({L}|{D})*            { yylval.str = yytext; return IDENTIFIER }
 
 \-?{D}+(\.{D}*)?         { num, err := strconv.ParseFloat(yytext, 32);
                            if (err != nil && err.(*strconv.NumError).Err == strconv.ErrSyntax) {

rules/lexer.l.go

@@ -411,7 +411,7 @@ var yyrules []yyrule = []yyrule{{regexp.MustCompile("[^\\n]"), nil, []yystartcon
 		return yyactionreturn{DURATION, yyRT_USER_RETURN}
 	}
 	return yyactionreturn{0, yyRT_FALLTHROUGH}
-}}, {regexp.MustCompile("([a-zA-Z_:])(([a-zA-Z_:])|([0-9]))+"), nil, []yystartcondition{}, false, func() (yyar yyactionreturn) {
+}}, {regexp.MustCompile("([a-zA-Z_:])(([a-zA-Z_:])|([0-9]))*"), nil, []yystartcondition{}, false, func() (yyar yyactionreturn) {
 	defer func() {
 		if r := recover(); r != nil {
 			if r != "yyREJECT" {

rules/rules_test.go

@@ -179,6 +179,13 @@ var expressionTests = []struct {
 		},
 		fullRanges:     8,
 		intervalRanges: 0,
+	}, {
+		expr: "x{y='testvalue'}",
+		output: []string{
+			"x{y='testvalue'} => 100 @[%v]",
+		},
+		fullRanges:     0,
+		intervalRanges: 1,
 		// Invalid expressions that should fail to parse.
 	}, {
 		expr:       "",

rules/testdata.go

@@ -140,6 +140,14 @@ var testMatrix = ast.Matrix{
 		},
 		Values: getTestValueStream(0, 800, 80),
 	},
+	// Single-letter metric and label names.
+	{
+		Metric: model.Metric{
+			model.MetricNameLabel: "x",
+			"y":                   "testvalue",
+		},
+		Values: getTestValueStream(0, 100, 10),
+	},
 }
 
 var testVector = getTestVectorFromTestMatrix(testMatrix)