diff --git a/proc/readproc.c b/proc/readproc.c
index d6bdbc9a..db90a2e9 100644
--- a/proc/readproc.c
+++ b/proc/readproc.c
@@ -78,8 +78,10 @@ static unsigned long long unhex(const char *restrict cp){
     unsigned long long ull = 0;
     for(;;){
         char c = *cp++;
-        if(unlikely(c<0x30)) break;
-        ull = (ull<<4) | (c - (c>0x57) ? 0x57 : 0x30) ;
+        if(!( (c >= '0' && c <= '9') ||
+              (c >= 'A' && c <= 'F') ||
+              (c >= 'a' && c <= 'f') )) break;
+        ull = (ull<<4) | (c - (c >= 'a' ? 'a'-10 : c >= 'A' ? 'A'-10 : '0'));
     }
     return ull;
 }