> For the complete documentation index, see [llms.txt](https://docs.feathery.io/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.feathery.io/platform/build-forms/advanced-logic/examples/field-validation/complex-field-validation.md).

# Complex Field Validation

Sometimes the standard custom field validation feature is not powerful and flexible enough to do more complex validations.  For example, if you wish to apply a validation to a field that requires the use of a regular expression, only the advanced logic rule feature supports this.

In the example below, the FullName field is required to have at least two parts (first and last names).  A regular expression is employed.

```
// Require the FullName field to have both first and last names
// Note that Feathery makes the field FullName available as a global variable

if (!FullName.value.match('[a-zA-Z]+ [a-zA-Z]+')) {
  FullName.setError('Please enter your full name');
}
```


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