A validator implementation written in Kotlin.

Validator.kt

sealed interface Validator {
    fun validate(value: String): Int?
}

object RequiredValidator : Validator {
    override fun validate(value: String): Int? =
        if (value.isNotBlank()) null else ValidationError.Required
}

object EmailValidator : Validator {
    private val emailRegex = """[\w\d._%+-]+@[\w\d.-]+\.\w{2,}""".toRegex()

    override fun validate(value: String): Int? =
        if (emailRegex.matches(value)) null else ValidationError.InvalidEmail
}

data class LengthValidator(private val length: Int) : Validator {
    override fun validate(value: String): Int? =
        if (value.length >= length) null else ValidationError.TooShort
}

object NumericValidator : Validator {
    private val numberRegex = """^-?\d+([.,]\d+)?""".toRegex()

    override fun validate(value: String): Int? =
        if (numberRegex.matches(value)) null else ValidationError.NotNumeric
}

@JvmInline
value class ValidationError private constructor(@StringRes val value: Int) {

    companion object {
        val Required = create(R.string.validation_required)
        val InvalidEmail = create(R.string.validation_not_valid)
        val TooShort = create(R.string.validation_too_short)
        val NotNumeric = create(R.string.validation_not_numeric)

        private fun create(@StringRes res: Int): Int {
            return ValidationError(res).value
        }
    }
}