+# check that a field only has callsign characters in it
+sub is_callsign
+{
+ return $_[0] =~ /^(?:[A-Z]{1,2}\d+|\d[A-Z]\d+)[A-Z]+(?:-\d{1,2}|\/[A-Z0-9]+)?$/;
+}
+
+# check that a PC protocol field is valid text
+sub is_pctext
+{
+ return $_[0] =~ /^[\x09\x20-\xFF]+$/;
+}
+
+# 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 $_[0] =~ /^[\d]+$/;
+}