What is one disadvantage of pattern-matching field validation in the UI?

The chosen answer highlights a specific challenge associated with pattern-matching field validation in the user interface. This type of validation is often limited to individual widgets, meaning that for each field that requires validation, developers must configure the rules separately. This can lead to increased complexity in the user interface code and can make maintenance more cumbersome, as each widget's validation logic needs to be managed independently. As a result, it might be less efficient in scenarios where multiple fields share similar validation requirements, as developers must replicate the validation configuration for each relevant widget instead of applying a more streamlined, universal approach.

In contrast, other options either describe characteristics of the validation system that are not inherently disadvantages. For instance, the ability for automatic application to all widgets or the use of custom error messages can enhance usability when implemented thoughtfully. Additionally, the functionality without user input does not address a disadvantage but rather reflects how the validation operates before a user interaction. Hence, the per-widget configuration represents a more specific drawback in terms of the effort and complexity involved in the implementation of this validation method throughout a user interface.