time: match month and day names only when not followed immediately by a lower-case letter

Avoids seeing "Janet" as "Januaryet".

Fixes #6020.

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/12448044

src/pkg/time/format.go

@@ -102,6 +102,16 @@ const (
 // std0x records the std values for "01", "02", ..., "06".
 var std0x = [...]int{stdZeroMonth, stdZeroDay, stdZeroHour12, stdZeroMinute, stdZeroSecond, stdYear}
 
+// startsWithLowerCase reports whether the the string has a lower-case letter at the beginning.
+// Its purpose is to prevent matching strings like "Month" when looking for "Mon".
+func startsWithLowerCase(str string) bool {
+	if len(str) == 0 {
+		return false
+	}
+	c := str[0]
+	return 'a' <= c && c <= 'z'
+}
+
 // nextStdChunk finds the first occurrence of a std string in
 // layout and returns the text before, the std string, and the text after.
 func nextStdChunk(layout string) (prefix string, std int, suffix string) {
@@ -112,7 +122,9 @@ func nextStdChunk(layout string) (prefix string, std int, suffix string) {
 				if len(layout) >= i+7 && layout[i:i+7] == "January" {
 					return layout[0:i], stdLongMonth, layout[i+7:]
 				}
-				return layout[0:i], stdMonth, layout[i+3:]
+				if !startsWithLowerCase(layout[i+3:]) {
+					return layout[0:i], stdMonth, layout[i+3:]
+				}
 			}
 
 		case 'M': // Monday, Mon, MST
@@ -121,7 +133,9 @@ func nextStdChunk(layout string) (prefix string, std int, suffix string) {
 					if len(layout) >= i+6 && layout[i:i+6] == "Monday" {
 						return layout[0:i], stdLongWeekDay, layout[i+6:]
 					}
-					return layout[0:i], stdWeekDay, layout[i+3:]
+					if !startsWithLowerCase(layout[i+3:]) {
+						return layout[0:i], stdWeekDay, layout[i+3:]
+					}
 				}
 				if layout[i:i+3] == "MST" {
 					return layout[0:i], stdTZ, layout[i+3:]

src/pkg/time/time_test.go

@@ -413,6 +413,8 @@ var formatTests = []FormatTest{
 	{"am/pm", "3pm", "9pm"},
 	{"AM/PM", "3PM", "9PM"},
 	{"two-digit year", "06 01 02", "09 02 04"},
+	// Three-letter months and days must not be followed by lower-case letter.
+	{"Janet", "Hi Janet, the Month is January", "Hi Janet, the Month is February"},
 	// Time stamps, Fractional seconds.
 	{"Stamp", Stamp, "Feb  4 21:00:57"},
 	{"StampMilli", StampMilli, "Feb  4 21:00:57.012"},
@@ -505,6 +507,8 @@ var parseTests = []ParseTest{
 	// Leading zeros in other places should not be taken as fractional seconds.
 	{"zero1", "2006.01.02.15.04.05.0", "2010.02.04.21.00.57.0", false, false, 1, 1},
 	{"zero2", "2006.01.02.15.04.05.00", "2010.02.04.21.00.57.01", false, false, 1, 2},
+	// Month and day names only match when not followed by a lower-case letter.
+	{"Janet", "Hi Janet, the Month is January: Jan _2 15:04:05 2006", "Hi Janet, the Month is February: Feb  4 21:00:57 2010", false, true, 1, 0},
 
 	// Accept any number of fractional second digits (including none) for .999...
 	// In Go 1, .999... was completely ignored in the format, meaning the first two