CSV importer: validate line_filter

And another one ;)

Since regular expressions can be quite sophisticated I think we should validate entries for line_filter.

First of all this should happen while submitting the form:

• execute the regex
• check for result FALSE

I not sure how to implement the config validation - is there a preferred method?
I think this should / could be done in class CRM_Banking_Form_Configure (CRM/Banking/Form/Configure.php).

• CRM_Banking_Form_Configure inherits CRM_Core_Form which offers public function validate(). Should I overwrite this one (of course calling parent::validate())?
• The example in the Developer documentation uses addRules().
• If I implement the validation in CRM_Banking_Form_Configure: how do I check which plugin is used?
• Or is there another way I am not aware of?

I will try to find out myself but I'm also grateful for any directions.

I think it would be best to also check the result during the actual imports - just in case someone fiddled with the JSON directly in the database table civicrm_bank_plugin_instance). What do you think?
In that case we could set a civi error - but I am not quite sure how do to that - maybe someone can give me a hint?

Start with CRM/Banking/Form/Configure.php, CRM_Core_Form::validate(), and the QuickForm addRules() example linked in the issue. Trace how the CSV importer uses line_filter and how configuration is read from civicrm_bank_plugin_instance. Done means invalid regular expressions are rejected during form submission and safely handled during imports.