diff --git a/lib/Makefile.am b/lib/Makefile.am
index c402ff02a3..3312662b04 100644
--- a/lib/Makefile.am
+++ b/lib/Makefile.am
@@ -186,14 +186,14 @@ libshadow_la_SOURCES = \
 	spawn.c \
 	sssd.c \
 	sssd.h \
-	string/ctype/strchrisascii/strchriscntrl.c \
-	string/ctype/strchrisascii/strchriscntrl.h \
-	string/ctype/strisascii/strisdigit.c \
-	string/ctype/strisascii/strisdigit.h \
-	string/ctype/strisascii/strisprint.c \
-	string/ctype/strisascii/strisprint.h \
-	string/ctype/strtoascii/strtolower.c \
-	string/ctype/strtoascii/strtolower.h \
+	string/ctype/isascii.c \
+	string/ctype/isascii.h \
+	string/ctype/strchrisascii.c \
+	string/ctype/strchrisascii.h \
+	string/ctype/strisascii.c \
+	string/ctype/strisascii.h \
+	string/ctype/strtoascii.c \
+	string/ctype/strtoascii.h \
 	string/memset/memzero.c \
 	string/memset/memzero.h \
 	string/sprintf/aprintf.c \
diff --git a/lib/chkname.c b/lib/chkname.c
index 0abee4d294..57588220fb 100644
--- a/lib/chkname.c
+++ b/lib/chkname.c
@@ -33,8 +33,9 @@
 
 #include "defines.h"
 #include "chkname.h"
-#include "string/ctype/strchrisascii/strchriscntrl.h"
-#include "string/ctype/strisascii/strisdigit.h"
+#include "string/ctype/isascii.h"
+#include "string/ctype/strchrisascii.h"
+#include "string/ctype/strisascii.h"
 #include "string/strcmp/streq.h"
 #include "string/strcmp/strcaseeq.h"
 
@@ -68,8 +69,8 @@ is_valid_name(const char *name)
 	 || streq(name, "..")
 	 || strspn(name, "-")
 	 || strpbrk(name, " \"#',/:;")
-	 || strchriscntrl(name)
-	 || strisdigit(name))
+	 || strchriscntrl_c(name)
+	 || strisdigit_c(name))
 	{
 		errno = EINVAL;
 		return false;
@@ -87,26 +88,16 @@ 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;
 		}
diff --git a/lib/fields.c b/lib/fields.c
index 144e5fd09a..b8e3bb9e8a 100644
--- a/lib/fields.c
+++ b/lib/fields.c
@@ -17,8 +17,8 @@
 #include <string.h>
 
 #include "prototypes.h"
-#include "string/ctype/strisascii/strisprint.h"
-#include "string/ctype/strchrisascii/strchriscntrl.h"
+#include "string/ctype/strisascii.h"
+#include "string/ctype/strchrisascii.h"
 #include "string/strcmp/streq.h"
 #include "string/strspn/stpspn.h"
 #include "string/strspn/stprspn.h"
@@ -41,12 +41,12 @@ valid_field_(const char *field, const char *illegal)
 
 	if (strpbrk(field, illegal))
 		return -1;
-	if (strchriscntrl(field))
+	if (strchriscntrl_c(field))
 		return -1;
-	if (strisprint(field))
-		return 0;
 	if (streq(field, ""))
 		return 0;
+	if (strisprint_c(field))
+		return 0;
 
 	return 1;  // !ASCII
 }
diff --git a/lib/getrange.c b/lib/getrange.c
index a7f8f05963..a7dd844f67 100644
--- a/lib/getrange.c
+++ b/lib/getrange.c
@@ -13,6 +13,7 @@
 #include "atoi/a2i.h"
 #include "defines.h"
 #include "prototypes.h"
+#include "string/ctype/isascii.h"
 #include "string/strcmp/streq.h"
 
 
@@ -57,7 +58,7 @@ getrange(const char *range,
 		if (streq(end, ""))
 			return 0;  /* <long>- */
 parse_max:
-		if (!isdigit((unsigned char) *end))
+		if (!isdigit_c(*end))
 			return -1;
 
 		if (a2ul(max, end, NULL, 10, *min, ULONG_MAX) == -1)
diff --git a/lib/obscure.c b/lib/obscure.c
index c572b96a34..485bfd2407 100644
--- a/lib/obscure.c
+++ b/lib/obscure.c
@@ -19,7 +19,7 @@
 #include "prototypes.h"
 #include "defines.h"
 #include "getdef.h"
-#include "string/ctype/strtoascii/strtolower.h"
+#include "string/ctype/strtoascii.h"
 #include "string/memset/memzero.h"
 #include "string/sprintf/aprintf.h"
 #include "string/strcmp/streq.h"
diff --git a/lib/port.c b/lib/port.c
index 357be218cc..b12d7d282c 100644
--- a/lib/port.c
+++ b/lib/port.c
@@ -19,6 +19,7 @@
 #include "defines.h"
 #include "port.h"
 #include "prototypes.h"
+#include "string/ctype/isascii.h"
 #include "string/strcmp/streq.h"
 #include "string/strcmp/strprefix.h"
 #include "string/strtok/stpsep.h"
@@ -211,7 +212,7 @@ getportent(void)
 		 * week or the other two values.
 		 */
 
-		for (i = 0; isalpha(cp[i]) && ('\0' != cp[i + 1]); i += 2) {
+		for (i = 0; isalpha_c(cp[i]) && ('\0' != cp[i + 1]); i += 2) {
 			switch ((cp[i] << 8) | (cp[i + 1])) {
 			case ('S' << 8) | 'u':
 				port.pt_times[j].t_days |= 01;
@@ -260,7 +261,7 @@ getportent(void)
 		 * representing the times of day.
 		 */
 
-		for (dtime = 0; isdigit (cp[i]); i++) {
+		for (dtime = 0; isdigit_c(cp[i]); i++) {
 			dtime = dtime * 10 + cp[i] - '0';
 		}
 
@@ -270,7 +271,7 @@ getportent(void)
 		port.pt_times[j].t_start = dtime;
 		cp = cp + i + 1;
 
-		for (dtime = 0, i = 0; isdigit (cp[i]); i++) {
+		for (dtime = 0, i = 0; isdigit_c(cp[i]); i++) {
 			dtime = dtime * 10 + cp[i] - '0';
 		}
 
diff --git a/lib/setupenv.c b/lib/setupenv.c
index 263221636d..f109e10afc 100644
--- a/lib/setupenv.c
+++ b/lib/setupenv.c
@@ -105,7 +105,7 @@ static void read_env_file (const char *filename)
 		} else if (*cp == '\0') {
 			/* end of string */
 			goto finished;
-		} else if (isspace (*cp)) {
+		} else if (isspace_c(*cp)) {
 			/* unescaped whitespace - end of string */
 			stpcpy(cp, "");
 			goto finished;
diff --git a/lib/string/README b/lib/string/README
index 9fbcbd842c..6ad0e1f5c8 100644
--- a/lib/string/README
+++ b/lib/string/README
@@ -69,21 +69,26 @@ Specific guidelines:
 
 ctype/ - Character classification and conversion functions
 
-    strchrisascii/
-	The functions defined under this directory
+    isascii.h
+	The functions defined in this file
+	return true
+	if the character
+	belongs to the category specified in the function name.
+
+    strchrisascii.h
+	The functions defined in this file
 	return true
 	if the string has any characters that
 	belong to the category specified in the function name.
 
-    strisascii/
-	The functions defined under this directory
+    strisascii.h
+	The functions defined in this file
 	return true
 	if all of the characters of the string
-	belong to the category specified in the function name
-	and the string is not an empty string.
+	belong to the category specified in the function name.
 
-    strtoascii/
-	The functions defined under this directory
+    strtoascii.h
+	The functions defined in this file
 	translate all characters in a string.
 
 memset/ - Memory zeroing
diff --git a/lib/string/ctype/isascii.c b/lib/string/ctype/isascii.c
new file mode 100644
index 0000000000..5409883124
--- /dev/null
+++ b/lib/string/ctype/isascii.c
@@ -0,0 +1,7 @@
+// SPDX-FileCopyrightText: 2024-2025, Alejandro Colomar <alx@kernel.org>
+// SPDX-License-Identifier: BSD-3-Clause
+
+
+#include "config.h"
+
+#include "string/ctype/isascii.h"
diff --git a/lib/string/ctype/isascii.h b/lib/string/ctype/isascii.h
new file mode 100644
index 0000000000..d0f59fbd58
--- /dev/null
+++ b/lib/string/ctype/isascii.h
@@ -0,0 +1,50 @@
+// SPDX-FileCopyrightText: 2024-2025, Alejandro Colomar <alx@kernel.org>
+// SPDX-License-Identifier: BSD-3-Clause
+
+
+#ifndef SHADOW_INCLUDE_LIB_STRING_CTYPE_ISASCII_H_
+#define SHADOW_INCLUDE_LIB_STRING_CTYPE_ISASCII_H_
+
+
+#include "config.h"
+
+#include <string.h>
+
+#include "string/strcmp/streq.h"
+
+
+#define CTYPE_CNTRL_C                                                      \
+	"\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_PFCHAR_C  CTYPE_ALNUM_C "._-"  // portable filename character set
+
+
+// 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 ispfchar_c(c)  (!streq(strchrnul(CTYPE_PFCHAR_C, c), ""))
+
+
+#endif  // include guard
diff --git a/lib/string/ctype/strisascii/strisprint.c b/lib/string/ctype/strchrisascii.c
similarity index 52%
rename from lib/string/ctype/strisascii/strisprint.c
rename to lib/string/ctype/strchrisascii.c
index 47fb2a3bfe..905430628a 100644
--- a/lib/string/ctype/strisascii/strisprint.c
+++ b/lib/string/ctype/strchrisascii.c
@@ -4,9 +4,4 @@
 
 #include "config.h"
 
-#include "string/ctype/strisascii/strisprint.h"
-
-#include <stdbool.h>
-
-
-extern inline bool strisprint(const char *s);
+#include "string/ctype/strchrisascii.h"
diff --git a/lib/string/ctype/strchrisascii.h b/lib/string/ctype/strchrisascii.h
new file mode 100644
index 0000000000..6d1c4ff022
--- /dev/null
+++ b/lib/string/ctype/strchrisascii.h
@@ -0,0 +1,20 @@
+// SPDX-FileCopyrightText: 2024-2025, Alejandro Colomar <alx@kernel.org>
+// SPDX-License-Identifier: BSD-3-Clause
+
+
+#ifndef SHADOW_INCLUDE_LIB_STRING_CTYPE_STRCHRISASCII_H_
+#define SHADOW_INCLUDE_LIB_STRING_CTYPE_STRCHRISASCII_H_
+
+
+#include "config.h"
+
+#include <string.h>
+
+#include "string/ctype/isascii.h"
+
+
+// strchriscntrl_c - string character is [:cntrl:] C-locale
+#define strchriscntrl_c(s)  (!!strpbrk(s, CTYPE_CNTRL_C))
+
+
+#endif  // include guard
diff --git a/lib/string/ctype/strchrisascii/strchriscntrl.c b/lib/string/ctype/strchrisascii/strchriscntrl.c
deleted file mode 100644
index ca77b4cfba..0000000000
--- a/lib/string/ctype/strchrisascii/strchriscntrl.c
+++ /dev/null
@@ -1,12 +0,0 @@
-// SPDX-FileCopyrightText: 2024, Alejandro Colomar <alx@kernel.org>
-// SPDX-License-Identifier: BSD-3-Clause
-
-
-#include "config.h"
-
-#include "string/ctype/strchrisascii/strchriscntrl.h"
-
-#include <stdbool.h>
-
-
-extern inline bool strchriscntrl(const char *s);
diff --git a/lib/string/ctype/strchrisascii/strchriscntrl.h b/lib/string/ctype/strchrisascii/strchriscntrl.h
deleted file mode 100644
index a1eaca371e..0000000000
--- a/lib/string/ctype/strchrisascii/strchriscntrl.h
+++ /dev/null
@@ -1,36 +0,0 @@
-// SPDX-FileCopyrightText: 2024-2025, Alejandro Colomar <alx@kernel.org>
-// SPDX-License-Identifier: BSD-3-Clause
-
-
-#ifndef SHADOW_INCLUDE_LIB_STRING_CTYPE_STRCHRISASCII_STRCHRISCNTRL_H_
-#define SHADOW_INCLUDE_LIB_STRING_CTYPE_STRCHRISASCII_STRCHRISCNTRL_H_
-
-
-#include "config.h"
-
-#include <ctype.h>
-#include <stdbool.h>
-
-#include "string/strcmp/streq.h"
-
-
-inline bool strchriscntrl(const char *s);
-
-
-// string character is [:cntrl:]
-// Return true if any iscntrl(3) character is found in the string.
-inline bool
-strchriscntrl(const char *s)
-{
-	for (; !streq(s, ""); s++) {
-		unsigned char  c = *s;
-
-		if (iscntrl(c))
-			return true;
-	}
-
-	return false;
-}
-
-
-#endif  // include guard
diff --git a/lib/string/ctype/strisascii.c b/lib/string/ctype/strisascii.c
new file mode 100644
index 0000000000..24e8a9cbb4
--- /dev/null
+++ b/lib/string/ctype/strisascii.c
@@ -0,0 +1,7 @@
+// SPDX-FileCopyrightText: 2024-2025, Alejandro Colomar <alx@kernel.org>
+// SPDX-License-Identifier: BSD-3-Clause
+
+
+#include "config.h"
+
+#include "string/ctype/strisascii.h"
diff --git a/lib/string/ctype/strisascii.h b/lib/string/ctype/strisascii.h
new file mode 100644
index 0000000000..eb4d831bbd
--- /dev/null
+++ b/lib/string/ctype/strisascii.h
@@ -0,0 +1,21 @@
+// SPDX-FileCopyrightText: 2024-2025, Alejandro Colomar <alx@kernel.org>
+// SPDX-License-Identifier: BSD-3-Clause
+
+
+#ifndef SHADOW_INCLUDE_LIB_STRING_CTYPE_STRISASCII_H_
+#define SHADOW_INCLUDE_LIB_STRING_CTYPE_STRISASCII_H_
+
+
+#include "config.h"
+
+#include "string/ctype/isascii.h"
+#include "string/strcmp/streq.h"
+#include "string/strspn/stpspn.h"
+
+
+// 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), "")
+
+
+#endif  // include guard
diff --git a/lib/string/ctype/strisascii/strisdigit.c b/lib/string/ctype/strisascii/strisdigit.c
deleted file mode 100644
index acd4522882..0000000000
--- a/lib/string/ctype/strisascii/strisdigit.c
+++ /dev/null
@@ -1,12 +0,0 @@
-// SPDX-FileCopyrightText: 2024, Alejandro Colomar <alx@kernel.org>
-// SPDX-License-Identifier: BSD-3-Clause
-
-
-#include "config.h"
-
-#include "string/ctype/strisascii/strisdigit.h"
-
-#include <stdbool.h>
-
-
-extern inline bool strisdigit(const char *s);
diff --git a/lib/string/ctype/strisascii/strisdigit.h b/lib/string/ctype/strisascii/strisdigit.h
deleted file mode 100644
index 0c5175fc8d..0000000000
--- a/lib/string/ctype/strisascii/strisdigit.h
+++ /dev/null
@@ -1,32 +0,0 @@
-// SPDX-FileCopyrightText: 2024, Alejandro Colomar <alx@kernel.org>
-// SPDX-License-Identifier: BSD-3-Clause
-
-
-#ifndef SHADOW_INCLUDE_LIB_STRING_CTYPE_STRISASCII_STRISDIGIT_H_
-#define SHADOW_INCLUDE_LIB_STRING_CTYPE_STRISASCII_STRISDIGIT_H_
-
-
-#include "config.h"
-
-#include <stdbool.h>
-
-#include "string/strcmp/streq.h"
-#include "string/strspn/stpspn.h"
-
-
-inline bool strisdigit(const char *s);
-
-
-// string is [:digit:]
-// Like isdigit(3), but check all characters in the string.
-inline bool
-strisdigit(const char *s)
-{
-	if (streq(s, ""))
-		return false;
-
-	return streq(stpspn(s, "0123456789"), "");
-}
-
-
-#endif  // include guard
diff --git a/lib/string/ctype/strisascii/strisprint.h b/lib/string/ctype/strisascii/strisprint.h
deleted file mode 100644
index 566dbf886d..0000000000
--- a/lib/string/ctype/strisascii/strisprint.h
+++ /dev/null
@@ -1,39 +0,0 @@
-// SPDX-FileCopyrightText: 2024-2025, Alejandro Colomar <alx@kernel.org>
-// SPDX-License-Identifier: BSD-3-Clause
-
-
-#ifndef SHADOW_INCLUDE_LIB_STRING_CTYPE_STRISASCII_STRISPRINT_H_
-#define SHADOW_INCLUDE_LIB_STRING_CTYPE_STRISASCII_STRISPRINT_H_
-
-
-#include "config.h"
-
-#include <ctype.h>
-#include <stdbool.h>
-
-#include "string/strcmp/streq.h"
-
-
-inline bool strisprint(const char *s);
-
-
-// string is [:print:]
-// Like isprint(3), but check all characters in the string.
-inline bool
-strisprint(const char *s)
-{
-	if (streq(s, ""))
-		return false;
-
-	for (; !streq(s, ""); s++) {
-		unsigned char  c = *s;
-
-		if (!isprint(c))
-			return false;
-	}
-
-	return true;
-}
-
-
-#endif  // include guard
diff --git a/lib/string/ctype/strtoascii/strtolower.c b/lib/string/ctype/strtoascii.c
similarity index 78%
rename from lib/string/ctype/strtoascii/strtolower.c
rename to lib/string/ctype/strtoascii.c
index 99ea4097ca..1e89acaaf5 100644
--- a/lib/string/ctype/strtoascii/strtolower.c
+++ b/lib/string/ctype/strtoascii.c
@@ -4,7 +4,7 @@
 
 #include "config.h"
 
-#include "string/ctype/strtoascii/strtolower.h"
+#include "string/ctype/strtoascii.h"
 
 
 extern inline char *strtolower(char *str);
diff --git a/lib/string/ctype/strtoascii/strtolower.h b/lib/string/ctype/strtoascii.h
similarity index 61%
rename from lib/string/ctype/strtoascii/strtolower.h
rename to lib/string/ctype/strtoascii.h
index d283fe1e87..c0c3be7177 100644
--- a/lib/string/ctype/strtoascii/strtolower.h
+++ b/lib/string/ctype/strtoascii.h
@@ -2,8 +2,8 @@
 // SPDX-License-Identifier: BSD-3-Clause
 
 
-#ifndef SHADOW_INCLUDE_LIB_STRING_CTYPE_STRTOASCII_STRTOLOWER_H_
-#define SHADOW_INCLUDE_LIB_STRING_CTYPE_STRTOASCII_STRTOLOWER_H_
+#ifndef SHADOW_INCLUDE_LIB_STRING_CTYPE_STRTOASCII_H_
+#define SHADOW_INCLUDE_LIB_STRING_CTYPE_STRTOASCII_H_
 
 
 #include "config.h"
@@ -16,9 +16,7 @@
 inline char *strtolower(char *str);
 
 
-// string convert-to lower-case
-// Like tolower(3), but convert all characters in the string.
-// Returns the input pointer.
+// strtolower - string convert-to lower-case
 inline char *
 strtolower(char *str)
 {
diff --git a/lib/subordinateio.c b/lib/subordinateio.c
index acd3f1ffdc..5ed5288fbb 100644
--- a/lib/subordinateio.c
+++ b/lib/subordinateio.c
@@ -22,7 +22,7 @@
 #include "alloc/malloc.h"
 #include "alloc/reallocf.h"
 #include "atoi/a2i.h"
-#include "string/ctype/strisascii/strisdigit.h"
+#include "string/ctype/strisascii.h"
 #include "string/sprintf/snprintf.h"
 #include "string/strcmp/streq.h"
 #include "string/strtok/strsep2arr.h"
@@ -938,7 +938,7 @@ static int append_uids(uid_t **uids, const char *owner, int n)
 	int    i;
 	uid_t  owner_uid;
 
-	if (strisdigit(owner)) {
+	if (strisdigit_c(owner)) {
 		i = sscanf(owner, "%d", &owner_uid);
 		if (i != 1) {
 			// should not happen
diff --git a/src/newusers.c b/src/newusers.c
index e9353fdc0a..6ac29c673d 100644
--- a/src/newusers.c
+++ b/src/newusers.c
@@ -56,6 +56,7 @@
 #include "shadow/gshadow/sgrp.h"
 #include "shadowlog.h"
 #include "sssd.h"
+#include "string/ctype/isascii.h"
 #include "string/sprintf/snprintf.h"
 #include "string/strcmp/streq.h"
 #include "string/strdup/strdup.h"
@@ -248,7 +249,7 @@ static int add_group (const char *name, const char *gid, gid_t *ngid, uid_t uid)
 		return 0;
 	}
 
-	if (isdigit (gid[0])) {
+	if (isdigit_c(gid[0])) {
 		/*
 		 * The GID is a number, which means either this is a brand
 		 * new group, or an existing group.
@@ -292,7 +293,7 @@ static int add_group (const char *name, const char *gid, gid_t *ngid, uid_t uid)
 	/*
 	 * Now I have all of the fields required to create the new group.
 	 */
-	if (!streq(gid, "") && (!isdigit(gid[0]))) {
+	if (!streq(gid, "") && (!isdigit_c(gid[0]))) {
 		grent.gr_name = xstrdup (gid);
 	} else {
 		grent.gr_name = xstrdup (name);
@@ -357,7 +358,7 @@ static int get_user_id (const char *uid, uid_t *nuid) {
 	 * The first guess for the UID is either the numerical UID that the
 	 * caller provided, or the next available UID.
 	 */
-	if (isdigit (uid[0])) {
+	if (isdigit_c(uid[0])) {
 		if ((get_uid(uid, nuid) == -1) || (*nuid == (uid_t)-1)) {
 			fprintf (stderr,
 			         _("%s: invalid user ID '%s'\n"),