Editing Field validate.pl (section)

== Example ==
The following example checks if the value(s) of a metadata field:
# Are 'loosely' URLs if the field type is [[API:EPrints/MetaField/Url|EPrints::MetaField::URL]].
# Have both a non-empty <tt>family</tt> and <tt>given</tt> sub-field if the field is of type [[API:EPrints/MetaField/Name|EPrints::MetaField::Name]].
# Are email addresses if the field type is [[API:EPrints/MetaField/Email|EPrints::MetaField::Email]].
# Are no longer than the <tt>maxlength</tt> attribute for the field.
# <tt>username</tt> is being changed on a [[API:EPrints/DataObj/User|EPrints::DataObj::User]] data object and this will lead it to have the same username as another user. (Unless EPrints <tt>allow_duplicate_usernames</tt> configuration allows this).
 $c->{validate_field} = sub
 {
   my( $field, $value, $repository, $for_archive ) = @_;
 
   my $xml = $repository->xml();
 
   # only apply checks if the value is set
   return () if !EPrints::Utils::is_set( $value );
 
   my @problems = ();
 
   # CHECKS IN HERE
 
   my $values = ref($value) eq "ARRAY" ? $value : [$value];
 
   # closure for generating the field link fragment
   my $f_fieldname = sub {
     my $f = defined $field->property( "parent" ) ? $field->property( "parent" ) : $field;
     my $fieldname = $xml->create_element( "span", class=>"ep_problem_field:".$f->get_name );
     $fieldname->appendChild( $f->render_name( $repository ) );
     return $fieldname;
   };
 
   # Loop over actual individual values to check URLs, names and emails
   foreach my $v (@$values)
   {
     next unless EPrints::Utils::is_set( $v );
 
     if( $field->isa( "EPrints::MetaField::Url" ) )
     {
       # Valid URI check (very loose)
       if( $v !~ /^\w+:/ )
       {
         push @problems, $repository->html_phrase( "validate:missing_http", fieldname=>&$f_fieldname );
       }
     }
     elsif( $field->isa( "EPrints::MetaField::Name" ) )
     {
       # Check a name has a family part
       if( !EPrints::Utils::is_set( $v->{family} ) )
       {
         push @problems, $repository->html_phrase( "validate:missing_family", fieldname=>&$f_fieldname );
       }
       # Check a name has a given part
       elsif( !EPrints::Utils::is_set( $v->{given} ) )
       {
         push @problems, $repository->html_phrase( "validate:missing_given", fieldname=>&$f_fieldname );
       }
     }
     elsif( $field->isa( "EPrints::MetaField::Email" ) )
     {
       # Check an email looks "ok". Just checks it has only one "@" and no
       # spaces.
       if( $v !~ /^[^ \@]+\@[^ \@]+$/ )
       {
         push @problems,$repository->html_phrase( "validate:bad_email", fieldname=>&$f_fieldname );
       }
     }
 
     # Check for overly long values
     # Applies to all subclasses of Id: Text, Longtext, Url etc.
     if( $field->isa( "EPrints::MetaField::Id" ) )
     {
       if( EPrints::Utils::is_set( $v ) && length($v) > $field->property( "maxlength" ) )
       {
         push @problems, $repository->html_phrase( "validate:truncated", fieldname=>&$f_fieldname );
       }
     }
 
     # Unless duplicate usernames are explicitly permitted do not allow username to be changed to that of an existing user
     if ( $field->get_name eq "username" && $field->dataset->id eq "user" && defined $repository->param( 'dataobj' ) && !$repository->config( "allow_duplicate_usernames" ) )
     {
       my $user_with_username = EPrints::DataObj::User::user_with_username( $repository, $v );
       my $current_user = $field->dataset->dataobj( $repository->param( 'dataobj' ) );
       if ( $user_with_username->id ne $current_user->id )
       {
         push @problems, $repository->html_phrase( "validate:duplicate_username", username => $repository->make_text( $v ) );
       }
     }
   }
 
   return( @problems );
 };