You should be able to create own validation via js.

https://blog.kyp.fr/how-to-validate-magento-configuration-values-format/

Anyway I found some solutions :) :
https://magento.stackexchange.com/questions/75342/using-ajax-validation-for-system-configuration-fields

http://stackoverflow.com/questions/16664396/how-to-validate-admin-config-options-for-magento-module

You should be able to create own validation via js.

https://blog.kyp.fr/how-to-validate-magento-configuration-values-format/

Anyway I found some solutions :) :
https://magento.stackexchange.com/questions/75342/using-ajax-validation-for-system-configuration-fields

https://stackoverflow.com/questions/16664396/how-to-validate-admin-config-options-for-magento-module

You should be able to create own validation via js.

https://blog.kyp.fr/how-to-validate-magento-configuration-values-format/

Anyway I found some solutions :) :
http://magento.stackexchange.com/questions/75342/using-ajax-validation-for-system-configuration-fields

http://stackoverflow.com/questions/16664396/how-to-validate-admin-config-options-for-magento-module

You should be able to create own validation via js.

https://blog.kyp.fr/how-to-validate-magento-configuration-values-format/

Anyway I found some solutions :) :
https://magento.stackexchange.com/questions/75342/using-ajax-validation-for-system-configuration-fields

http://stackoverflow.com/questions/16664396/how-to-validate-admin-config-options-for-magento-module