+sub is_prefix
+{
+ return $_[0] =~ m!^(?:[A-Z]{1,2}\d+ | \d[A-Z]{1,2}\d+)!x # basic prefix
+}
+
+
+# check that a PC protocol field is valid text
+sub is_pctext
+{
+ return undef unless length $_[0];
+ return undef if $_[0] =~ /[\x00-\x08\x0a-\x1f\x80-\x9f]/;
+ return 1;
+}
+
+# check that a PC prot flag is fairly valid (doesn't check the difference between 1/0 and */-)
+sub is_pcflag
+{
+ return $_[0] =~ /^[01\*\-]+$/;
+}
+
+# check that a thing is a frequency
+sub is_freq
+{
+ return $_[0] =~ /^\d+(?:\.\d+)?$/;
+}
+
+# check that a thing is just digits
+sub is_digits
+{
+ return $_[0] =~ /^[\d]+$/;
+}