It has been explained to me that the "decimal digit" property has the
following meaning: "Decimal numbers are those using in decimal-radix
number systems. In particular, the sequence of the ONE character
followed by the TWO character is interpreted as having the value of twelve".
I don't agree with that explanation.
If I use isdigit() in c or a corresponding function in another language to check a character, I only expect to find out whether to not that character is or is not a decimal digit. I won't know whether it is being used as part of a decimal-radix number or not.
Many languages and applications allow use of a filter template such as "9999-999-9999999" or "####-####-#######" in which the figure "9" or "#" in the template must be filled by a decimal digit in the data.I mean, it's quite clearly ignored in sentences like "My phone number is 0044-1727-6000000", or "The codepoint of the space character is U+0020".
Allowing *only* decimal digits (and additional template characters) in a field is often useful. Keycodes and product codes often contain particular positions that must be alphabetic and other positions where only decimal numbers are allowed.
What possible use could any mechanical algorithm make of the "decimal
digit" property that it could not equally well make of the "digit" or
"numeric" properties?
We hardly want to allow Roman numeral characters in a field that we are going to evaluate as though it were decimal. If we are interpreting a field as a radix 10 number it is reasonable to validate the field as containing only radix 10 characters (and allowed numeric separators) proceeded or followed by spaces.
Generally a check on whether a character is a decimal digit is part of validation, whether validation of previously stored data or of data as it is being input. Of course we will probably normally want a tighter validation. We probably won't want to allow a number that is composed of mixed Latin, Arabic and Hindu digits even though it can be evaluated.
On the other hand, in a multi-lingual and multi-script environment it would be useful to ignore scripts in evaluating numbers just as one often ignores case in evaluating strings. Note that data is often supplied from a client in text format, say tab-delimited, with numbers in text format. It would be useful to verify such data by checking that the numbers are proper decimal numbers regardless of script before actually reading the data into another database where they might (or might not) be converted to binary format.
Checking for decimal numbers is also useful in parsing addresses which is a necessity for address validation and address correction software.
Jim Allan
