go-playground/validator

The go-playground/validator package is a Go library for validating the contents of data structures. The problem it solves is a common one in any application that accepts input: you need to check that data conforms to your expectations before processing it, that an email address actually looks like an email address, that a required field is not empty, that a number falls within an allowed range, or that two password fields match each other. In Go, data is typically organized into structs (groupings of named fields). This library lets you annotate those struct fields with simple tags in your source code, short labels like "required", "min=1", "max=100", "email", or "url", and then call a single validation function that checks all the rules at once. If any rule fails, the library returns detailed error information identifying exactly which field failed and why. Error messages can be translated into multiple languages. What makes this library particularly capable is its support for cross-field comparisons (checking that one field's value is greater than another's), deep validation inside slices, arrays, and maps (so you can validate every element of a list, not just the list itself), and the ability to define custom validation functions for your own business rules. It ships with a very large set of built-in validators covering string formats, network addresses, numbers, dates, geographic coordinates, financial identifiers like credit card numbers and IBAN codes, and much more. It is also the default validator used by the Gin web framework. You would add this library to any Go service or application that processes user input, form data, API request bodies, or configuration.