코드 :

function upper_case(str)
{
   regexp = /^[A-Z]/;
   if (regexp.test(str))
    {
      console.log("String's first character is uppercase");
    } 
    else
    {
      console.log("String's first character is not uppercase");
    }
}
upper_case('Abcd');
upper_case('abcd');

결과 : 

String's first character is uppercase

String's first character is not uppercase