Checker

Checker is a Go library that helps you validate user input. It can be used to validate user input stored in a struct, or to validate individual pieces of input.

There are many validation libraries available, but I prefer to build my own tools and avoid pulling in unnecessary dependencies. That's why I created Checker, a simple validation library with no dependencies. It's easy to use and gets the job done.

Usage

To get started, install the Checker library with the following command:

go get github.com/cinar/checker

Next, you will need to import the library into your source file. You can do this by following the example below:

import (
    "github.com/cinar/checker"
)

Validating User Input Stored in a Struct

Checker can be used in two ways. The first way is to validate user input stored in a struct. To do this, you can list the checkers through the struct tag for each field. Here is an example:

type Person struct {
    Name string `checkers:"required"`
}

person := &Person{}

mistakes, valid := checker.Check(person)
if !valid {
    // Send the mistakes back to the user
}

Validating Individual User Input

If you do not want to validate user input stored in a struct, you can individually call the checker functions to validate the user input. Here is an example:

var name

result := checker.IsRequired(name)
if result != ResultValid {
    // Send the result back to the user
}

Normalizers and Checkers

Checkers are used to check for problems in user input, while normalizers are used to transform user input into a preferred format. For example, a normalizer could be used to trim spaces from the beginning and end of a string, or to convert a string to title case.

I am not entirely happy with the decision to combine checkers and normalizers into a single library, but using them together can be useful. Normalizers and checkers can be mixed in any order when defining the validation steps for user data. For example, the trim normalizer can be used in conjunction with the required checker to first trim the user input and then check if the user provided the required information. Here is an example:

type Person struct {
    Name string `checkers:"trim required"`
}

Checkers Provided

This package currently provides the following checkers:

• alphanumeric checks if the given string consists of only alphanumeric characters.
• ascii checks if the given string consists of only ASCII characters.
• cidr checker checks if the value is a valid CIDR notation IP address and prefix length.
• credit-card checks if the given value is a valid credit card number.
• digits checks if the given string consists of only digit characters.
• email checks if the given string is an email address.
• fqdn checks if the given string is a fully qualified domain name.
• ip checks if the given value is an IP address.
• ipv4 checks if the given value is an IPv4 address.
• ipv6 checks if the given value is an IPv6 address.
• isbn checks if the given value is a valid ISBN number.
• luhn checks if the given number is valid based on the Luhn algorithm.
• mac checks if the given value is a valid an IEEE 802 MAC-48, EUI-48, EUI-64, or a 20-octet IP over InfiniBand link-layer address.
• max checks if the given value is less than the given maximum.
• max-length checks if the length of the given value is less than the given maximum length.
• min checks if the given value is greather than the given minimum.
• min-length checks if the length of the given value is greather than the given minimum length.
• regexp checks if the given string matches the regexp pattern.
• required checks if the required value is provided.
• same checks if the given value is equal to the value of the field with the given name.
• url checks if the given value is a valid URL.

Normalizers Provided

This package currently provides the following normalizers. They can be mixed with the checkers when defining the validation steps for user data.

• html-escape applies HTML escaping to special characters.
• html-unescape applies HTML unescaping to special characters.
• lower maps all Unicode letters in the given value to their lower case.
• upper maps all Unicode letters in the given value to their upper case.
• title maps the first letter of each word to their upper case.
• trim removes the whitespaces at the beginning and at the end of the given value.
• trim-left removes the whitespaces at the beginning of the given value.
• trim-right removes the whitespaces at the end of the given value.
• url-escape applies URL escaping to special characters.
• url-unescape applies URL unescaping to special characters.

Custom Checkers

To define a custom checker, you need to create a new function with the following parameters:

func CustomChecker(value, parent reflect.Value) Result {
    return ResultValid
}

type MakeFunc You also need to create a make function that takes the checker configuration and returns a reference to the checker function.

func CustomMaker(params string) CheckFunc {
    return CustomChecker
}

Finally, you need to call the Register function to register your custom checker.

checker.Register("custom-checker", CustomMaker)

Once you have registered your custom checker, you can use it by simply specifying its name.

type User struct {
    Username string `checkers:"custom-checker"`
}

Contributing to the Project

Anyone can contribute to Checkers library. Please make sure to read our Contributor Covenant Code of Conduct guide first. Follow the How to Contribute to Checker to contribute.

License

This library is free to use, modify, and distribute under the terms of the MIT license. The full license text can be found in the LICENSE file.

The MIT license is a permissive license that allows you to do almost anything with the library, as long as you retain the copyright notice and the license text. This means that you can use the library in commercial products, modify it, and redistribute it without having to ask for permission from the authors.

The LICENSE file is located in the root directory of the library. You can open it in a text editor to read the full license text.