ash v1.51.2
ash

Ash.Resource.Validation.Builtins.match

You're seeing just the function match, go back to Ash.Resource.Validation.Builtins module for more information.
Link to this function

match(attribute, match, message \\ nil)

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Validates that an attribute's value matches a given regex or string, using the provided error, message if not.

String.match?/2 is used to determine if it matches.