diff --git a/rules/lexer.l b/rules/lexer.l
index 2972fa7be..8f4663029 100644
--- a/rules/lexer.l
+++ b/rules/lexer.l
@@ -9,6 +9,7 @@ import (
 
 D                       [0-9]
 L                       [a-zA-Z_:]
+U                       [smhdwy]
 
 %x S_COMMENTS
 
@@ -30,6 +31,7 @@ AVG|SUM|MAX|MIN          { yylval.str = yytext; return AGGR_OP }
 [+\-]                    { yylval.str = yytext; return ADDITIVE_OP }
 [*/%]                    { yylval.str = yytext; return MULT_OP }
 
+{D}+{U}                  { yylval.str = yytext; return DURATION }
 {L}({L}|{D})+            { yylval.str = yytext; return IDENTIFIER }
 
 \-?{D}+(\.{D}*)?         { num, err := strconv.ParseFloat(yytext, 32);
diff --git a/rules/lexer.l.go b/rules/lexer.l.go
index 3de45a3af..a6e6876af 100644
--- a/rules/lexer.l.go
+++ b/rules/lexer.l.go
@@ -397,6 +397,20 @@ var yyrules []yyrule = []yyrule{{regexp.MustCompile("[^\\n]"), nil, []yystartcon
 		return yyactionreturn{MULT_OP, yyRT_USER_RETURN}
 	}
 	return yyactionreturn{0, yyRT_FALLTHROUGH}
+}}, {regexp.MustCompile("([0-9])+([smhdwy])"), nil, []yystartcondition{}, false, func() (yyar yyactionreturn) {
+	defer func() {
+		if r := recover(); r != nil {
+			if r != "yyREJECT" {
+				panic(r)
+			}
+			yyar.returnType = yyRT_REJECT
+		}
+	}()
+	{
+		yylval.str = yytext
+		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) {
 	defer func() {
 		if r := recover(); r != nil {
diff --git a/rules/parser.y b/rules/parser.y
index 10f32ae12..69b1d093c 100644
--- a/rules/parser.y
+++ b/rules/parser.y
@@ -22,7 +22,7 @@
    */
 %token START_RULES START_EXPRESSION
 
-%token <str> IDENTIFIER STRING
+%token <str> IDENTIFIER STRING DURATION
 %token <num> NUMBER
 %token PERMANENT GROUP_OP
 %token <str> AGGR_OP CMP_OP ADDITIVE_OP MULT_OP
@@ -100,7 +100,7 @@ rule_expr          : '(' rule_expr ')'
                        $$, err = NewFunctionCall($1, []ast.Node{})
                        if err != nil { yylex.Error(err.Error()); return 1 }
                      }
-		   | rule_expr '[' STRING ']'
+                   | rule_expr '[' DURATION ']'
                      {
                        var err error
                        $$, err = NewMatrix($1, $3)
@@ -112,7 +112,7 @@ rule_expr          : '(' rule_expr ')'
                        $$, err = NewVectorAggregation($1, $3, $5)
                        if err != nil { yylex.Error(err.Error()); return 1 }
                      }
-		   /* Yacc can only attach associativity to terminals, so we
+                   /* Yacc can only attach associativity to terminals, so we
                     * have to list all operators here. */
                    | rule_expr ADDITIVE_OP rule_expr
                      {
diff --git a/rules/parser.y.go b/rules/parser.y.go
index 05d4bea89..f1712842c 100644
--- a/rules/parser.y.go
+++ b/rules/parser.y.go
@@ -22,19 +22,21 @@ const START_RULES = 57346
 const START_EXPRESSION = 57347
 const IDENTIFIER = 57348
 const STRING = 57349
-const NUMBER = 57350
-const PERMANENT = 57351
-const GROUP_OP = 57352
-const AGGR_OP = 57353
-const CMP_OP = 57354
-const ADDITIVE_OP = 57355
-const MULT_OP = 57356
+const DURATION = 57350
+const NUMBER = 57351
+const PERMANENT = 57352
+const GROUP_OP = 57353
+const AGGR_OP = 57354
+const CMP_OP = 57355
+const ADDITIVE_OP = 57356
+const MULT_OP = 57357
 
 var yyToknames = []string{
 	"START_RULES",
 	"START_EXPRESSION",
 	"IDENTIFIER",
 	"STRING",
+	"DURATION",
 	"NUMBER",
 	"PERMANENT",
 	"GROUP_OP",
@@ -74,27 +76,27 @@ const yyLast = 84
 var yyAct = []int{
 
 	32, 36, 31, 16, 6, 17, 15, 16, 18, 40,
-	14, 14, 19, 46, 14, 54, 25, 26, 27, 8,
-	33, 10, 21, 38, 9, 20, 17, 15, 16, 17,
-	15, 16, 7, 30, 28, 14, 39, 22, 14, 8,
-	33, 10, 15, 16, 9, 48, 49, 21, 53, 8,
-	14, 10, 7, 58, 9, 57, 42, 37, 41, 43,
-	44, 47, 7, 45, 52, 13, 50, 24, 35, 59,
+	14, 14, 21, 46, 14, 20, 25, 26, 27, 8,
+	33, 54, 10, 38, 22, 9, 19, 17, 15, 16,
+	17, 15, 16, 7, 30, 28, 14, 8, 33, 14,
+	10, 15, 16, 9, 47, 48, 49, 21, 53, 14,
+	39, 7, 8, 37, 58, 10, 57, 42, 9, 41,
+	43, 44, 52, 13, 45, 35, 7, 24, 50, 59,
 	56, 37, 23, 2, 3, 11, 5, 4, 1, 12,
 	34, 51, 55, 29,
 }
 var yyPact = []int{
 
-	69, -1000, -1000, 43, 56, -1000, 17, 43, 6, 18,
-	-1000, -1000, 66, -1000, 60, 43, 43, 43, 14, -1000,
-	13, 51, 43, 31, -13, -11, -10, 29, -1000, 38,
-	-1000, -1000, 17, -1000, 42, -1000, -1000, 48, -7, 46,
-	-1000, -1000, 33, -1000, 65, 59, 54, 43, -1000, -1000,
-	-1000, -1000, -4, 17, 64, 35, -1000, -1000, 63, -1000,
+	69, -1000, -1000, 46, 53, -1000, 17, 46, -5, 4,
+	-1000, -1000, 66, -1000, 59, 46, 46, 46, 14, -1000,
+	13, 47, 46, 30, -14, -12, -11, 27, -1000, 38,
+	-1000, -1000, 17, -1000, 42, -1000, -1000, 48, -8, 28,
+	-1000, -1000, 31, -1000, 65, 61, 51, 46, -1000, -1000,
+	-1000, -1000, 1, 17, 64, 35, -1000, -1000, 63, -1000,
 }
 var yyPgo = []int{
 
-	0, 83, 82, 81, 1, 80, 12, 0, 2, 79,
+	0, 83, 82, 81, 1, 80, 26, 0, 2, 79,
 	78, 77, 76, 75,
 }
 var yyR1 = []int{
@@ -113,12 +115,12 @@ var yyR2 = []int{
 }
 var yyChk = []int{
 
-	-1000, -10, 4, 5, -11, -12, -7, 19, 6, 11,
-	8, -13, -9, 9, 21, 13, 14, 12, -7, -6,
-	19, 16, 19, 6, 7, -7, -7, -7, 20, -1,
-	20, -8, -7, 7, -5, 17, -4, 6, -7, -6,
-	22, 20, 18, 17, 18, 15, 20, 15, -8, -4,
-	7, -3, 10, -7, 19, -2, 6, 20, 18, 6,
+	-1000, -10, 4, 5, -11, -12, -7, 20, 6, 12,
+	9, -13, -9, 10, 22, 14, 15, 13, -7, -6,
+	20, 17, 20, 6, 8, -7, -7, -7, 21, -1,
+	21, -8, -7, 7, -5, 18, -4, 6, -7, -6,
+	23, 21, 19, 18, 19, 16, 21, 16, -8, -4,
+	7, -3, 11, -7, 20, -2, 6, 21, 19, 6,
 }
 var yyDef = []int{
 
@@ -135,20 +137,20 @@ var yyTok1 = []int{
 	3, 3, 3, 3, 3, 3, 3, 3, 3, 3,
 	3, 3, 3, 3, 3, 3, 3, 3, 3, 3,
 	3, 3, 3, 3, 3, 3, 3, 3, 3, 3,
-	19, 20, 3, 3, 18, 3, 3, 3, 3, 3,
+	20, 21, 3, 3, 19, 3, 3, 3, 3, 3,
 	3, 3, 3, 3, 3, 3, 3, 3, 3, 3,
-	3, 15, 3, 3, 3, 3, 3, 3, 3, 3,
+	3, 16, 3, 3, 3, 3, 3, 3, 3, 3,
 	3, 3, 3, 3, 3, 3, 3, 3, 3, 3,
 	3, 3, 3, 3, 3, 3, 3, 3, 3, 3,
-	3, 21, 3, 22, 3, 3, 3, 3, 3, 3,
+	3, 22, 3, 23, 3, 3, 3, 3, 3, 3,
 	3, 3, 3, 3, 3, 3, 3, 3, 3, 3,
 	3, 3, 3, 3, 3, 3, 3, 3, 3, 3,
-	3, 3, 3, 16, 3, 17,
+	3, 3, 3, 17, 3, 18,
 }
 var yyTok2 = []int{
 
 	2, 3, 4, 5, 6, 7, 8, 9, 10, 11,
-	12, 13, 14,
+	12, 13, 14, 15,
 }
 var yyTok3 = []int{
 	0,
diff --git a/rules/rules_test.go b/rules/rules_test.go
index 60da8f761..1893e34ec 100644
--- a/rules/rules_test.go
+++ b/rules/rules_test.go
@@ -105,7 +105,7 @@ var expressionTests = []struct {
 			"http_requests{job='app-server'} => 5200 @[%v]",
 		},
 	}, {
-		expr: "rate(http_requests['25m'])",
+		expr: "delta(http_requests[25m], 1)",
 		output: []string{
 			"http_requests{group='canary',instance='0',job='api-server'} => 150 @[%v]",
 			"http_requests{group='canary',instance='0',job='app-server'} => 350 @[%v]",
@@ -121,7 +121,10 @@ var expressionTests = []struct {
 		expr:       "",
 		shouldFail: true,
 	}, {
-		expr:       "http_requests[1m]",
+		expr:       "1 - http_requests",
+		shouldFail: true,
+	}, {
+		expr:       "http_requests['1m']",
 		shouldFail: true,
 	},
 }