JSFiddle - React, Tailwind, and code Playground

HTML

<form name="form1" action="#" method="post">
    Nome
    <input id="nome" type="text" name="nome">
    <br/> Email
    <input id="email" type="text" name="email">
    <br/> Telefone
    <input id="telefone" type="text" name="telefone">
    <br/>
    <input type="submit" value="testar" onclick="return validaForm(this.form)">
</form>

JavaScript

var regras = {
    nome: /^([a-zA-Zà-úÀ-Ú0-9]|-|_|\s)+$/,
    email: /^([\w-]+(?:\.[\w-]+)*)@((?:[\w-]+\.)*\w[\w-]{0,66})\.([a-z]{2,6}(?:\.[a-z]{2})?)$/i,
    telefone: /^[0-9]{1,}$/
}

function validaForm(form) {
    var invalidos = Object.keys(regras).map(function(nome) {
        var valida = form[nome].value.match(regras[nome]);
        if (!valida) form[nome].placeholder = ['Insira o', nome, 'corretamente'].join(' ');
        return valida;
    });
    return invalidos.length == 0;
}