Allow lower-case BY operator.

rules/lexer.l

@@ -38,7 +38,7 @@ U                       [smhdwy]
 \/\/[^\r\n]*\n           { /* gobble up one-line comments */ }
 
 permanent                { return PERMANENT }
-BY                       { return GROUP_OP }
+BY|by                    { return GROUP_OP }
 AVG|SUM|MAX|MIN          { yylval.str = yytext; return AGGR_OP }
 avg|sum|max|min          { yylval.str = strings.ToUpper(yytext); return AGGR_OP }
 \<|>|AND|OR              { yylval.str = yytext; return CMP_OP }

rules/lexer.l.go

@@ -315,7 +315,7 @@ var yyrules []yyrule = []yyrule{{regexp.MustCompile("[^\\n]"), nil, []yystartcon
 		return yyactionreturn{PERMANENT, yyRT_USER_RETURN}
 	}
 	return yyactionreturn{0, yyRT_FALLTHROUGH}
-}}, {regexp.MustCompile("BY"), nil, []yystartcondition{}, false, func() (yyar yyactionreturn) {
+}}, {regexp.MustCompile("BY|by"), nil, []yystartcondition{}, false, func() (yyar yyactionreturn) {
 	defer func() {
 		if r := recover(); r != nil {
 			if r != "yyREJECT" {

rules/rules_test.go

@@ -219,8 +219,11 @@ var expressionTests = []struct {
 		intervalRanges: 1,
 	}, {
 		// Lower-cased aggregation operators should work too.
-		expr:           "sum(http_requests) + min(http_requests) + max(http_requests) + avg(http_requests)",
-		output:         []string{"http_requests{} => 4950 @[%v]"},
+		expr:           "sum(http_requests) by (job) + min(http_requests) by (job) + max(http_requests) by (job) + avg(http_requests) by (job)",
+		output:					[]string{
+			"http_requests{job='app-server'} => 4550 @[%v]",
+			"http_requests{job='api-server'} => 1750 @[%v]",
+		},
 		fullRanges:     0,
 		intervalRanges: 8,
 	}, {