diff --git a/src/strutil.c b/src/strutil.c
index fcb12232..36c02281 100644
--- a/src/strutil.c
+++ b/src/strutil.c
@@ -241,9 +241,19 @@ int str_is_in_list(const char *s, const char **l, size_t nl, int (*f)(const char
   return 0;
 }
 
+// Is the string a decimal bus/device number that ends in a colon or nul?
+static int is_busdev_num(const char *s) {
+  const char *t = s;
+
+  while(*t >= '0' && *t <= '9')
+    t++;
+
+  return t > s && (*t == ':' || *t == 0);
+}
+
 // Do the two strings represent the same decimal number or are they the same up to a limiting colon?
 int str_busdev_eq(const char *s, const char *t) {
-  if(*s >= '0' && *s <='9' && *t >= '0' && *t <='9') // Both look like a decimal number
+  if(is_busdev_num(s) && is_busdev_num(t))
     if(strtoull(s, NULL, 10) == strtoull(t, NULL, 10))
       return 1;