diff --git a/libraries/AP_ADSB/AP_ADSB.cpp b/libraries/AP_ADSB/AP_ADSB.cpp
index dc0cb16b2ec8d..9844b3b444f72 100644
--- a/libraries/AP_ADSB/AP_ADSB.cpp
+++ b/libraries/AP_ADSB/AP_ADSB.cpp
@@ -39,6 +39,8 @@
 #include <GCS_MAVLink/GCS.h>
 #include <AP_RTC/AP_RTC.h>
 
+#include <ctype.h>
+
 #define VEHICLE_TIMEOUT_MS              5000   // if no updates in this time, drop it from the list
 #define ADSB_SQUAWK_OCTAL_DEFAULT       1200
 
@@ -778,8 +780,7 @@ void AP_ADSB::set_callsign(const char* str, const bool append_icao)
                 break;
             }
 
-        } else if (('A' <= str[i] && str[i] <= 'Z') ||
-                   ('0' <= str[i] && str[i] <= '9')) {
+        } else if (isupper(str[i]) || isdigit(str[i])) {
             // valid as-is
             // spaces are also allowed but are handled in the last else
             out_state.cfg.callsign[i] = str[i];
diff --git a/libraries/AP_CANManager/AP_SLCANIface.cpp b/libraries/AP_CANManager/AP_SLCANIface.cpp
index 496e8850ed2c1..d172451fe3b83 100644
--- a/libraries/AP_CANManager/AP_SLCANIface.cpp
+++ b/libraries/AP_CANManager/AP_SLCANIface.cpp
@@ -22,6 +22,7 @@
 #if AP_CAN_SLCAN_ENABLED
 #include <AP_HAL/AP_HAL.h>
 #include <AP_Common/AP_Common.h>
+#include <AP_Math/AP_Math.h>
 
 #include "AP_CANManager.h"
 
diff --git a/libraries/AP_HAL/utility/print_vprintf.cpp b/libraries/AP_HAL/utility/print_vprintf.cpp
index df7b3105364c1..776125581479d 100644
--- a/libraries/AP_HAL/utility/print_vprintf.cpp
+++ b/libraries/AP_HAL/utility/print_vprintf.cpp
@@ -39,6 +39,7 @@
 #include "print_vprintf.h"
 
 #include <cmath>
+#include <ctype.h>
 #include <stdarg.h>
 #include <string.h>
 
@@ -126,7 +127,7 @@ void print_vprintf(AP_HAL::BetterStream *s, const char *fmt, va_list ap)
                 }
 
                 if (flags < FL_LONG) {
-                    if (c >= '0' && c <= '9') {
+                    if (isdigit(c)) {
                         c -= '0';
                         if (flags & FL_PREC) {
                             prec = 10*prec + c;
diff --git a/libraries/AP_Math/definitions.h b/libraries/AP_Math/definitions.h
index c0280b54b9a23..ab4a57fd12057 100644
--- a/libraries/AP_Math/definitions.h
+++ b/libraries/AP_Math/definitions.h
@@ -1,6 +1,7 @@
 #pragma once
 
 #include <cmath>
+#include <ctype.h>
 
 #include <AP_HAL/AP_HAL_Boards.h>
 
@@ -127,3 +128,5 @@ static const double WGS84_E = (sqrt(2 * WGS84_F - WGS84_F * WGS84_F));
 
 // kg/m^3 to g/cm^3
 #define KG_PER_M3_TO_G_PER_CM3(x) (0.001 * x)
+
+#define ishexa(c)     (isdigit(c) || ((c) >= 'A' && (c) <= 'F') || ((c) >= 'a' && (c) <= 'f'))
diff --git a/libraries/AP_RangeFinder/AP_RangeFinder_LightWareI2C.cpp b/libraries/AP_RangeFinder/AP_RangeFinder_LightWareI2C.cpp
index 5e600ca463dd2..8c4b7b70c594d 100644
--- a/libraries/AP_RangeFinder/AP_RangeFinder_LightWareI2C.cpp
+++ b/libraries/AP_RangeFinder/AP_RangeFinder_LightWareI2C.cpp
@@ -16,6 +16,8 @@
 
 #if AP_RANGEFINDER_LWI2C_ENABLED
 
+#include <ctype.h>
+
 #include <AP_HAL/AP_HAL.h>
 #include <AP_HAL/utility/sparse-endian.h>
 
@@ -431,9 +433,7 @@ bool AP_RangeFinder_LightWareI2C::sf20_parse_stream(uint8_t *stream_buf,
     bool number_found = false;
     uint16_t accumulator = 0;
     uint16_t digit_u16 = (uint16_t)stream_buf[*p_num_processed_chars];
-    while ((((digit_u16 <= '9') &&
-             (digit_u16 >= '0')) ||
-            (digit_u16 == '.')) &&
+    while ((isdigit(digit_u16) || (digit_u16 == '.')) &&
            (*p_num_processed_chars < lx20_max_reply_len_bytes)) {
         (*p_num_processed_chars)++;
         if (decrement_multiplier) {
diff --git a/libraries/AP_Winch/AP_Winch_Daiwa.cpp b/libraries/AP_Winch/AP_Winch_Daiwa.cpp
index 88f6184472631..713548468291e 100644
--- a/libraries/AP_Winch/AP_Winch_Daiwa.cpp
+++ b/libraries/AP_Winch/AP_Winch_Daiwa.cpp
@@ -118,7 +118,7 @@ void AP_Winch_Daiwa::read_data_from_winch()
     while (nbytes-- > 0) {
         int16_t b = uart->read();
 
-        if ((b >= '0' && b <= '9') || (b >= 'A' && b <= 'F') || (b >= 'a' && b <= 'f')) {
+        if (ishexa(b)) {
             // add digits to buffer
             buff[buff_len] = b;
             buff_len++;