common: improve IsHexAddress and add tests (#15551)

Also unexport isHex, hasHexPrefix because IsHexAddress is the only caller.

Fixes #15550

common/bytes.go

@@ -17,9 +17,7 @@
 // Package common contains various helper functions.
 package common
 
-import (
-	"encoding/hex"
-)
+import "encoding/hex"
 
 func ToHex(b []byte) string {
 	hex := Bytes2Hex(b)
@@ -55,14 +53,24 @@ func CopyBytes(b []byte) (copiedBytes []byte) {
 	return
 }
 
-func HasHexPrefix(str string) bool {
-	l := len(str)
-	return l >= 2 && str[0:2] == "0x"
+func hasHexPrefix(str string) bool {
+	return len(str) >= 2 && str[0] == '0' && (str[1] == 'x' || str[1] == 'X')
 }
 
-func IsHex(str string) bool {
-	l := len(str)
-	return l >= 4 && l%2 == 0 && str[0:2] == "0x"
+func isHexCharacter(c byte) bool {
+	return ('0' <= c && c <= '9') || ('a' <= c && c <= 'f') || ('A' <= c && c <= 'F')
+}
+
+func isHex(str string) bool {
+	if len(str)%2 != 0 {
+		return false
+	}
+	for _, c := range []byte(str) {
+		if !isHexCharacter(c) {
+			return false
+		}
+	}
+	return true
 }
 
 func Bytes2Hex(d []byte) string {