Validation of the form (three first characters different from the space)

I do not know how to enter the validation in the folmula. I want the first 3 characters to be different from the space (sign = "").Example:surname is "xyz"- error. surname is "aaa aaaaa" - correctly. surname is"aa aaaaaa" - error.My code with which I tried.

`<input type="text" name="surname" class="input-medium flat has- 
placeholder" required="required" id="surname" data-validation- 
rule="minLength" data-validation-value="2" pattern="[A-Za-z]{3,25}+[" "]+ 
[A-Za-z]" aria-required="true" placeholder="surname">`