In a JSON schema the format keyword has a number of pre-defined formats (date, email, etc.) but can also be any custom format (see 7. Semantic Validation with "format"). If you simply need a to match a field against a regular expression you should use the pattern keyword instead. Custom formats are intended for annotation and more complex validation that is not possible to accomplish with a regex.
Initialize a new format with default values.
A short label that is displayed at the beginning of the input field.
A short label that is displayed at the end of the input field.
A short hint that describes the expected value of the field.
The browser is not permitted to automatically enter or select a value for this field.
The browser is allowed to automatically complete the input. No guidance is provided as to the type of data expected in the field, so the browser may use its own judgement.
The browser is allowed to automatically complete the input. See https://developer.mozilla.org/en-US/docs/Web/HTML/Attributes/autocomplete for a list of possible values and https://cloudfour.com/thinks/autofill-what-web-devs-should-know-but-dont/ for in-depth guidance on how to use autocomplete.
The type of input to use. The default is text.
The expected number of lines. If more than one the field will be rendered as a textarea. The default is 1.
Customize the input field with your own HTML.
A validation function (see etaque/elm-form for details on how to write a validation function).
A custom format.