✅ GoValidate — Struct Validation for Go

Tag-based validation with 20+ built-in rules, nested struct support, and custom rules — zero dependencies.

GoValidate provides declarative struct validation using Go struct tags. Supports nested structs, slices, custom rules, and JSON field names — all with zero external dependencies.

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📦 Features

Feature	Description
Struct Tags	validate:"required,min=3,max=100"
20+ Built-in Rules	required, min, max, email, url, ip, alpha, regex...
Nested Structs	Recursive validation with dot-notation errors
Slice Validation	Validates each element: Items[0].Name
Custom Rules	RegisterRule("phone", fn)
JSON Names	Uses json tag name in error messages
Error Map	errs.Map() → map[string][]string
Cross-field	oneof, numeric comparisons
Zero Dependencies	Only Go standard library + reflect

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🚀 Quick Start

go get github.com/kzxl/govalidate

package main

import (
    "fmt"
    "github.com/kzxl/govalidate"
)

type User struct {
    Name     string `json:"name"     validate:"required,min=3,max=50"`
    Email    string `json:"email"    validate:"required,email"`
    Age      int    `json:"age"      validate:"gte=0,lte=150"`
    Password string `json:"password" validate:"required,min=8"`
    Role     string `json:"role"     validate:"oneof=admin user moderator"`
}

func main() {
    user := User{
        Name:  "Jo",          // too short
        Email: "invalid",     // bad email
        Age:   -1,            // negative
    }

    errs := govalidate.Validate(user)
    if errs.HasErrors() {
        for _, e := range errs.Errors {
            fmt.Printf("  %s [%s]: %s\n", e.Field, e.Rule, e.Message)
        }
        // Output:
        //   name [min]: must be at least 3 characters
        //   email [email]: must be a valid email address
        //   age [gte]: must be >= 0
        //   password [required]: is required
    }

    // Error map for API responses
    fmt.Println(errs.Map())
    // map[name:[must be at least 3 characters] email:[must be a valid email] ...]
}

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📖 Built-in Rules

Rule	Example	Description
required	validate:"required"	Field must not be zero value
min	validate:"min=3"	Min length (string/slice) or min value (number)
max	validate:"max=100"	Max length or value
len	validate:"len=5"	Exact length
gt / gte	validate:"gt=0"	Greater than / greater than or equal
lt / lte	validate:"lte=100"	Less than / less than or equal
email	validate:"email"	Valid email format
url	validate:"url"	Valid URL with scheme and host
ip	validate:"ip"	Valid IPv4 or IPv6
alpha	validate:"alpha"	Letters only
alphanum	validate:"alphanum"	Letters and numbers only
numeric	validate:"numeric"	Numeric characters only
lowercase	validate:"lowercase"	Must be all lowercase
uppercase	validate:"uppercase"	Must be all uppercase
contains	validate:"contains=keyword"	Must contain substring
startswith	validate:"startswith=Hello"	Must start with prefix
endswith	validate:"endswith=!"	Must end with suffix
oneof	validate:"oneof=admin user"	Must be one of listed values
regex	validate:"regex=^\\d{3}$"	Must match regex pattern
datetime	validate:"datetime=2006-01-02"	Must parse with Go layout

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🔧 Custom Rules

govalidate.RegisterRule("phone", func(v reflect.Value, param string) string {
    if v.Kind() != reflect.String { return "" }
    if !phoneRegex.MatchString(v.String()) {
        return "must be a valid phone number"
    }
    return ""
})

type Contact struct {
    Phone string `validate:"required,phone"`
}

---

🏗️ Nested Struct & Slice Validation

type Order struct {
    Customer Customer  `validate:"required"`
    Items    []Item    `validate:"min=1"`
}

type Item struct {
    Name  string `validate:"required"`
    Price int    `validate:"gt=0"`
}

// Errors:
// customer.name: is required
// Items[0].Price: must be > 0

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📄 License

Apache License 2.0 — See LICENSE for details.

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✅ Validate structs, the Go way.