Eiffel regular expression validation

How do you create a regular expression for a certain string? And can you do it in the Assertion (precondition part of the code)?

I've been google-ing around but couldn't get anything convincing.

The question is like this:

Add a precondition to the DEPARTMENT (the class that we're working on) creation procedure that ensures that the phone number is valid. There are three possible valid phone number formats. A valid phone number consists of one of:

eight digits, the first of which is non-zero
a leading zero, a single non-zero digit area code, and then eight digits, the first of which is non-zero
a leading ‘+’, followed by a two digit country code, then a single non-zero digit area code, and then eight digits, the first of which is non-zero

Any embedded spaces are to be ignored when validating a phone number.

It is acceptable, but not required, to add a PHONE_NUMBER class to the system as part of solving this problem.

Solution

There are several different questions to be answered:

How to check if a given string matches a specified regular expression in Eiffel? One can use a class RX_PCRE_MATCHER from the Gobo library. The feature compile allows setting the required regular expression and the feature recognizes allows testing if the string matches it.
How to write a regular expression for the given phone number specification? Something like "(|0[1-9]|\+[0-9]{2}[1-9])[1-9][0-8]{7}" should do though I have not checked it. It's possible to take intermediate white spaces into account in the regular expression itself, but it's much easier to get rid of them before passing to the regular expression matcher by applying prune_all (' ') on the input string.
How to add a precondition to a creation procedure to verify that the argument satisfies it? Let's assume that from the previous items we constructed a function is_phone_number that takes a STRING and returns a BOOLEAN that indicates if the specified string represents a valid phone number. A straightforward solution would be to write
```
make (tel: STRING)
    require
        is_phone_number (tel)
    ...
```
and have a feature is_phone_number in the class DEPARTMENT itself. But this prevents us from checking if the specified string represents a phone number before calling this creation procedure. So it makes sense to move is_phone_number to the class PHONE_NUMBER_VALIDATOR that class DEPARTMENT will inherit. Similarly, if PHONE_NUMBER needs to validate the string against specified rules, it can inherit PHONE_NUMBER_VALIDATOR and reuse the feature is_phone_number.