Moose Attribute constraint: Regex OR undef

hoppfrosch has asked for the wisdom of the Perl Monks concerning the following question:

I want to use Moose type constraints for an attribute which could either be undefined or - if the attribute is provided on new() - match a given RegEx.

What I currently have is:

has 'attr' => (
    is => 'ro',
    isa => subtype(
      as 'Str',
      where { /$regEx/ }
    ),
    required => 0
);
[download]

I cannot figure out, where to put the undef to - or I have an incorrect syntax. Any hint welcome ...

Comment on Moose Attribute constraint: Regex OR undef Download Code

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Re: Moose Attribute constraint: Regex OR undef by 1nickt (Canon) on Oct 31, 2016 at 12:57 UTC
Maybe you need `Maybe`? `subtype 'MyMatch', as 'Str', where { /$regEx/ }, message { "$_ didn't match" }; has 'attr' => ( is => 'ro', isa => 'Maybe[MyMatch]', required => 0 );` [download] See doc: The Maybe[`a] type deserves a special mention. Used by itself, it doesn't really mean anything (and is equivalent to Item). When it is parameterized, it means that the value is either undef or the parameterized type. So Maybe[Int] means an integer or undef. [download] Hope this helps! The way forward always starts with a minimal test.	[reply] [d/l] [select]
Re^2: Moose Attribute constraint: Regex OR undef by hoppfrosch (Scribe) on Nov 01, 2016 at 09:26 UTC
Silly me: I was close to the same solution - but I failed due to the usage of round brackets instead of square brackets with `Maybe[....]` Thanks!	[reply] [d/l]
Re: Moose Attribute constraint: Regex OR undef by Corion (Patriarch) on Oct 31, 2016 at 12:19 UTC
Can you show us a short program that shows the intended usage of your class? Also, what errors do you currently get (or not)? I assume that `where` takes a subroutine reference which gets the thing to test in `$_`, so maybe the following already is enough? `... where { not defined $_ or $_ =~ /$regEx/ }` [download] Update: Negated the expression, thanks to 1nickt	[reply] [d/l] [select]