All field keywords beginning with html_form_ are passed directly into the form hash. Thus, they are really defined in form.tt which ships with Gantry.
html_form_
html_form_constraint specifies a Data::FormValidator constraint which will be checked with either AutoCRUD or CRUD. The simplest constraints are regexes, but anything Data::FormValidator understands will work. Note that you need backticks around the value in the bigtop file, but inside them you should have unquoted perl code.
html_form_constraint
To see an example, build:
bigtop -c example.bigtop all
Change to the newly created Kids directory and look in the form method in lib/Kids/GEN/Child.pm.
To install Bigtop, copy and paste the appropriate command in to your terminal.
cpanm
cpanm Bigtop
CPAN shell
perl -MCPAN -e shell install Bigtop
For more information on module installation, please visit the detailed CPAN module installation guide.