Integrate the library into you're project:

go get github.com/go-playground/validator/v10

Define a struct and annotate fields with validation rules. The following snippet creates a User struct and requires the Name field to be non‑empty.

package main

import (
    "log"
    "github.com/go-playground/validator/v10"
)

type User struct {
    Name string `validate:"required"`
}

func main() {
    v := validator.New()
    u := User{Name: "Alice"}

    if err := v.Struct(u); err != nil {
        log.Fatalf("validation issue: %s", err)
    }
    log.Println("struct valid")
}

Tag Composition Rules

• Comma (,) – Separates multiple rules; validated left to right. No spaces around the comma.
• Dash (-) – Excludes the field from validation entirely.
• Pipe (|) – Acts as an OR condition; at least one rule must pass.

Common Valiadtion Attributes

Universal

• required – Rejects the field's zero value.
• omitempty – Skips subsequent checks when the field holds its zero value.

Strings

• min=n, max=n, len=n – Length constraints.
• alpha, alphanum, numeric – Character set restrictions.
• email, url, uuid – Well‑known formats.
• datetime=format – Time format match, e.g., datetime=2006-01-02.
• eq=value, ne=value – Equality comparisons (no quotes needed).
• oneof=opt1 opt2 opt3 – Allowed values.
• contains=sub, excludes=sub, startswith=pref, endswith=suf – Substring tests.

Numbers

• gt=n, gte=n, lt=n, lte=n – Relational comparisons.
• eq=n, ne=n – Equality checks.
• oneof=v1 v2 v3 – Enumerated values.

Slices & Arrays

• min=n, max=n, len=n – Length boundaries.
• unique – Enforces distinct elements.

Maps

• Surround key rules with keys / endkeys. Combine with dive for deep validation. Example: dive,keys,min=1,max=100,endkeys,required,gt=0

Deep Validation (dive)

• Instructs the validator to inspect elements inside slices, arrays, and maps.
• For a []string field: validate:"dive,min=2" ensures each string has at least two characters.
• For a []Struct field: validate:"dive" evaluates struct-level tags automatically (no extra dive needed for nested structs).
• Use structonly on the parent field to bypass child struct validation.

Cross‑Field Comparisons

• eqfield=OtherField – Values must be equal.
• nefield=OtherField – Values must differ.
• gtefield=OtherField, ltefield=OtherField – Ordering constraints.

Example of cross‑field usage:

type Range struct {
    Start int `validate:"ltefield=End"`
    End   int
}