diff --git a/lib/chkname.c b/lib/chkname.c
index ee8ff9d108..262ca811ec 100644
--- a/lib/chkname.c
+++ b/lib/chkname.c
@@ -75,26 +75,16 @@ is_valid_name(const char *name)
 	 * sake of Samba 3.x "add machine script"
 	 */
 
-	if (!((*name >= 'a' && *name <= 'z') ||
-	      (*name >= 'A' && *name <= 'Z') ||
-	      (*name >= '0' && *name <= '9') ||
-	      *name == '_' ||
-	      *name == '.'))
-	{
+	if (!ispfchar_c(*name)) {
 		errno = EILSEQ;
 		return false;
 	}
 
 	while (!streq(++name, "")) {
-		if (!((*name >= 'a' && *name <= 'z') ||
-		      (*name >= 'A' && *name <= 'Z') ||
-		      (*name >= '0' && *name <= '9') ||
-		      *name == '_' ||
-		      *name == '.' ||
-		      *name == '-' ||
-		      streq(name, "$")
-		     ))
-		{
+		if (streq(name, "$"))  // Samba
+			return true;
+
+		if (!ispfchar_c(*name)) {
 			errno = EILSEQ;
 			return false;
 		}
@@ -146,7 +136,7 @@ is_valid_domain_label(const char *label)
 		errno = EINVAL;
 		return false;
 	}
-	if (!streq(stpspn(label, CTYPE_ALNUM_C "-"), "")) {
+	if (!strisldh_rfc1035_c(label)) {
 		errno = EINVAL;
 		return false;
 	}
diff --git a/lib/string/ctype/isascii.h b/lib/string/ctype/isascii.h
index 6500717c6a..750289632f 100644
--- a/lib/string/ctype/isascii.h
+++ b/lib/string/ctype/isascii.h
@@ -1,4 +1,4 @@
-// SPDX-FileCopyrightText: 2024-2025, Alejandro Colomar <alx@kernel.org>
+// SPDX-FileCopyrightText: 2024-2026, Alejandro Colomar <alx@kernel.org>
 // SPDX-License-Identifier: BSD-3-Clause
 
 
@@ -19,41 +19,46 @@
 	"\x1F\x1E\x1D\x1C\x1B\x1A\x19\x18\x17\x16\x15\x14\x13\x12\x11\x10" \
 	"\x0F\x0E\x0D\x0C\x0B\x0A\x09\x08\x07\x06\x05\x04\x03\x02\x01" /*NUL*/
 
-#define CTYPE_LOWER_C     "abcdefghijklmnopqrstuvwxyz"
-#define CTYPE_UPPER_C     "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
-#define CTYPE_DIGIT_C     "0123456789"
-#define CTYPE_PUNCT_C     "!\"#$%&'()*+,-./:;<=>?@[\\]^_`{|}~"
-#define CTYPE_SPACE_C     " \t\n\v\f\r"
-#define CTYPE_ALPHA_C     CTYPE_LOWER_C CTYPE_UPPER_C
-#define CTYPE_ALNUM_C     CTYPE_ALPHA_C CTYPE_DIGIT_C
-#define CTYPE_GRAPH_C     CTYPE_ALNUM_C CTYPE_PUNCT_C
-#define CTYPE_PRINT_C     CTYPE_GRAPH_C " "
-#define CTYPE_XDIGIT_C    CTYPE_DIGIT_C "abcdefABCDEF"
-#define CTYPE_ASCII_C     CTYPE_PRINT_C CTYPE_CNTRL_C /*NUL*/
+#define CTYPE_LOWER_C          "abcdefghijklmnopqrstuvwxyz"
+#define CTYPE_UPPER_C          "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
+#define CTYPE_DIGIT_C          "0123456789"
+#define CTYPE_PUNCT_C          "!\"#$%&'()*+,-./:;<=>?@[\\]^_`{|}~"
+#define CTYPE_SPACE_C          " \t\n\v\f\r"
+#define CTYPE_ALPHA_C          CTYPE_LOWER_C CTYPE_UPPER_C
+#define CTYPE_ALNUM_C          CTYPE_ALPHA_C CTYPE_DIGIT_C
+#define CTYPE_GRAPH_C          CTYPE_ALNUM_C CTYPE_PUNCT_C
+#define CTYPE_PRINT_C          CTYPE_GRAPH_C " "
+#define CTYPE_XDIGIT_C         CTYPE_DIGIT_C "abcdefABCDEF"
+#define CTYPE_ASCII_C          CTYPE_PRINT_C CTYPE_CNTRL_C /*NUL*/
+#define CTYPE_PFCHAR_C         CTYPE_ALNUM_C "._-"  // portable filename character set
+#define CTYPE_LDH_RFC1035_C    CTYPE_ALNUM_C "-"  // letter, digit, hyphen
 
 
 // isascii_c - is [:ascii:] C-locale
-#define isascii_c(c)      (!!strchr(CTYPE_ASCII_C, c))
-#define iscntrl_c(c)      (!!strchr(CTYPE_CNTRL_C, c))
-#define islower_c(c)      (!streq(strchrnul(CTYPE_LOWER_C, c), ""))
-#define isupper_c(c)      (!streq(strchrnul(CTYPE_UPPER_C, c), ""))
-#define isdigit_c(c)      (!streq(strchrnul(CTYPE_DIGIT_C, c), ""))
-#define ispunct_c(c)      (!streq(strchrnul(CTYPE_PUNCT_C, c), ""))
-#define isspace_c(c)      (!streq(strchrnul(CTYPE_SPACE_C, c), ""))
-#define isalpha_c(c)      (!streq(strchrnul(CTYPE_ALPHA_C, c), ""))
-#define isalnum_c(c)      (!streq(strchrnul(CTYPE_ALNUM_C, c), ""))
-#define isgraph_c(c)      (!streq(strchrnul(CTYPE_GRAPH_C, c), ""))
-#define isprint_c(c)      (!streq(strchrnul(CTYPE_PRINT_C, c), ""))
-#define isxdigit_c(c)     (!streq(strchrnul(CTYPE_XDIGIT_C, c), ""))
+#define isascii_c(c)           (!!strchr(CTYPE_ASCII_C, c))
+#define iscntrl_c(c)           (!!strchr(CTYPE_CNTRL_C, c))
+#define islower_c(c)           (!streq(strchrnul(CTYPE_LOWER_C, c), ""))
+#define isupper_c(c)           (!streq(strchrnul(CTYPE_UPPER_C, c), ""))
+#define isdigit_c(c)           (!streq(strchrnul(CTYPE_DIGIT_C, c), ""))
+#define ispunct_c(c)           (!streq(strchrnul(CTYPE_PUNCT_C, c), ""))
+#define isspace_c(c)           (!streq(strchrnul(CTYPE_SPACE_C, c), ""))
+#define isalpha_c(c)           (!streq(strchrnul(CTYPE_ALPHA_C, c), ""))
+#define isalnum_c(c)           (!streq(strchrnul(CTYPE_ALNUM_C, c), ""))
+#define isgraph_c(c)           (!streq(strchrnul(CTYPE_GRAPH_C, c), ""))
+#define isprint_c(c)           (!streq(strchrnul(CTYPE_PRINT_C, c), ""))
+#define isxdigit_c(c)          (!streq(strchrnul(CTYPE_XDIGIT_C, c), ""))
+#define ispfchar_c(c)          (!streq(strchrnul(CTYPE_PFCHAR_C, c), ""))
+#define isldh_rfc1035_c(c)     (!streq(strchrnul(CTYPE_LDH_RFC1035_C, c), ""))
 
 
 // strisascii_c - string is [:ascii:] C-locale
-#define strisdigit_c(s)   streq(stpspn(s, CTYPE_DIGIT_C), "")
-#define strisprint_c(s)   streq(stpspn(s, CTYPE_PRINT_C), "")
+#define strisdigit_c(s)        streq(stpspn(s, CTYPE_DIGIT_C), "")
+#define strisprint_c(s)        streq(stpspn(s, CTYPE_PRINT_C), "")
+#define strisldh_rfc1035_c(s)  streq(stpspn(s, CTYPE_LDH_RFC1035_C), "")
 
 
 // strchriscntrl_c - string character is [:cntrl:] C-locale
-#define strchriscntrl_c(s)  (!!strpbrk(s, CTYPE_CNTRL_C))
+#define strchriscntrl_c(s)     (!!strpbrk(s, CTYPE_CNTRL_C))
 
 
 #endif  // include guard