Tighten restrictions on float decoding (#171)

ParseFloat() accepts strings that contain digits with a single 'e'
character somewhere in the middle as valid floats. The YAML spec does
not accept these.

This causes problems especially when dealing with short commit hashes,
e.g. `123456e1`

decode_test.go

@@ -581,6 +581,15 @@ var unmarshalTests = []struct {
 		"\xfe\xff\x00\xf1\x00o\x00\xf1\x00o\x00:\x00 \x00v\x00e\x00r\x00y\x00 \x00y\x00e\x00s\x00 \xd8=\xdf\xd4\x00\n",
 		M{"ñoño": "very yes 🟔"},
 	},
+
+	// YAML Float regex shouldn't match this
+	{
+		"a: 123456e1\n",
+		M{"a": "123456e1"},
+	}, {
+		"a: 123456E1\n",
+		M{"a": "123456E1"},
+	},
 }
 
 type M map[interface{}]interface{}

resolve.go

@@ -3,6 +3,7 @@ package yaml
 import (
 	"encoding/base64"
 	"math"
+	"regexp"
 	"strconv"
 	"strings"
 	"unicode/utf8"
@@ -80,6 +81,8 @@ func resolvableTag(tag string) bool {
 	return false
 }
 
+var yamlStyleFloat = regexp.MustCompile(`^[-+]?[0-9]*\.?[0-9]+([eE][-+][0-9]+)?$`)
+
 func resolve(tag string, in string) (rtag string, out interface{}) {
 	if !resolvableTag(tag) {
 		return tag, in
@@ -135,10 +138,12 @@ func resolve(tag string, in string) (rtag string, out interface{}) {
 			if err == nil {
 				return yaml_INT_TAG, uintv
 			}
+			if yamlStyleFloat.MatchString(plain) {
 				floatv, err := strconv.ParseFloat(plain, 64)
 				if err == nil {
 					return yaml_FLOAT_TAG, floatv
 				}
+			}
 			if strings.HasPrefix(plain, "0b") {
 				intv, err := strconv.ParseInt(plain[2:], 2, 64)
 				if err == nil {