I propose that an official slug validation should be added to string().

Slugs are human readable strings that are suitable for use in URLs (https://en.wikipedia.org/wiki/Clean_URL#Slug). They are used everywhere from blogs to online stores to disambiguate their URLs. (eg: https://myblog.com/posts/15 versus https://mystore.com/posts/my-post-title-as-a-slug )

The string() validator already has several specific validations. Some of these include common form types like email and url, as well as many types of IDs such as uuid and cuid. Slugs span both of these categories since they behave like a unique ID, but they're also usually entered by hand in a form field.

Usually, a slug is only letters and numbers, all lowercase and a single dash used for spaces. Sometimes underscores are allowed.

The use case could be

string().slug() // no underscores allowed by default

// VALID
'test'
'test-test'
'01'
'01-test'

// INVALID
''
'TEST'
'te$t'
'-test'
'test-'
'test_test'

string().slug({ underscores: true })

// VALID
'test_test'