diff --git a/arch/x86/tools/gen-insn-attr-x86.awk b/arch/x86/tools/gen-insn-attr-x86.awk
index 7a6850683c34dcce220ecbf970c053c3f141d4bd..eaf11f52fc0bd8e74513ad8e83a65cb37c1ba3cc 100644
--- a/arch/x86/tools/gen-insn-attr-x86.awk
+++ b/arch/x86/tools/gen-insn-attr-x86.awk
@@ -6,8 +6,6 @@
 
 # Awk implementation sanity check
 function check_awk_implement() {
-	if (!match("abc", "[[:lower:]]+"))
-		return "Your awk doesn't support charactor-class."
 	if (sprintf("%x", 0) != "0")
 		return "Your awk has a printf-format problem."
 	return ""
@@ -44,12 +42,12 @@ BEGIN {
 	delete gtable
 	delete atable
 
-	opnd_expr = "^[[:alpha:]/]"
+	opnd_expr = "^[A-Za-z/]"
 	ext_expr = "^\\("
 	sep_expr = "^\\|$"
-	group_expr = "^Grp[[:alnum:]]+"
+	group_expr = "^Grp[0-9A-Za-z]+"
 
-	imm_expr = "^[IJAO][[:lower:]]"
+	imm_expr = "^[IJAO][a-z]"
 	imm_flag["Ib"] = "INAT_MAKE_IMM(INAT_IMM_BYTE)"
 	imm_flag["Jb"] = "INAT_MAKE_IMM(INAT_IMM_BYTE)"
 	imm_flag["Iw"] = "INAT_MAKE_IMM(INAT_IMM_WORD)"
@@ -62,7 +60,7 @@ BEGIN {
 	imm_flag["Ob"] = "INAT_MOFFSET"
 	imm_flag["Ov"] = "INAT_MOFFSET"
 
-	modrm_expr = "^([CDEGMNPQRSUVW/][[:lower:]]+|NTA|T[012])"
+	modrm_expr = "^([CDEGMNPQRSUVW/][a-z]+|NTA|T[012])"
 	force64_expr = "\\([df]64\\)"
 	rex_expr = "^REX(\\.[XRWB]+)*"
 	fpu_expr = "^ESC" # TODO