diff --git a/lib/Makefile.am b/lib/Makefile.am
index e76e7446a..7eb2ee296 100644
--- a/lib/Makefile.am
+++ b/lib/Makefile.am
@@ -176,6 +176,8 @@ libshadow_la_SOURCES = \
 	spawn.c \
 	sssd.c \
 	sssd.h \
+	string/ctype/strisascii_c/strisdigit_c.c \
+	string/ctype/strisascii_c/strisdigit_c.h \
 	string/memset/memzero.c \
 	string/memset/memzero.h \
 	string/sprintf/snprintf.c \
diff --git a/lib/string/ctype/strisascii_c/strisdigit_c.c b/lib/string/ctype/strisascii_c/strisdigit_c.c
new file mode 100644
index 000000000..65562593b
--- /dev/null
+++ b/lib/string/ctype/strisascii_c/strisdigit_c.c
@@ -0,0 +1,12 @@
+// SPDX-FileCopyrightText: 2024, Alejandro Colomar <alx@kernel.org>
+// SPDX-License-Identifier: BSD-3-Clause
+
+
+#include <config.h>
+
+#include "string/ctype/strisascii_c/strisdigit_c.h"
+
+#include <stdbool.h>
+
+
+extern inline bool strisdigit_c(const char *s);
diff --git a/lib/string/ctype/strisascii_c/strisdigit_c.h b/lib/string/ctype/strisascii_c/strisdigit_c.h
new file mode 100644
index 000000000..fe7e496c3
--- /dev/null
+++ b/lib/string/ctype/strisascii_c/strisdigit_c.h
@@ -0,0 +1,32 @@
+// SPDX-FileCopyrightText: 2024, Alejandro Colomar <alx@kernel.org>
+// SPDX-License-Identifier: BSD-3-Clause
+
+
+#ifndef SHADOW_INCLUDE_LIB_STRING_CTYPE_STRISASCII_C_STRISDIGIT_C_H_
+#define SHADOW_INCLUDE_LIB_STRING_CTYPE_STRISASCII_C_STRISDIGIT_C_H_
+
+
+#include <config.h>
+
+#include <stdbool.h>
+
+#include "string/strchr/stpspn.h"
+#include "string/strcmp/streq.h"
+
+
+inline bool strisdigit_c(const char *s);
+
+
+// Like isdigit(3),
+// but check all characters in the string, and use the C locale.
+inline bool
+strisdigit_c(const char *s)
+{
+	if (streq(s, ""))
+		return false;
+
+	return streq(stpspn(s, "0123456789"), "");
+}
+
+
+#endif  // include guard