title: How can i use Joi valid function and allow both lowercase and uppercase letters
body: <p>Hello guys and thank you for your time.
  I am trying to create a validation middleware.
  I am using Joi to validate the information i receive from the request body.</p>
  
  <p>i would like to allow only the following strings for example: ['a','b','c'] but also allow both upper case and lowercase instances. (for example 'A' is also valid);</p>
  
  <p>what i tried to do:</p>
  
  <pre><code>const schema = Joi.object().keys({
     letter: Joi.string().valid('a','b','c').insensitive()
  })
  
  schema.validate({letter: 'B'}) //return error but i want it to return 'B'
  </code></pre>
  
  <p>I have also tried every combination of lowercase and uppercase functions with convert but i could not make it work.
  i would like that for example {letter: 'A'} would turn out valid.</p>
  
  <p>is there a way to combine valid function with insensitive so that uppercase and lowercase will all get accepted?</p>
  
  <p>Thank you for your time.</p>