Kata

Write a function to test whether a given input is a valid email address.

For the purposes of this kata, here is what makes a valid email:

• At least one letter character at the beginning
• All characters between the first character and the @ (if any) must be letters, numbers, or underscores
• There must be an @ character (after the points listed just now)
• After the @ character, there must be at least one word character (letter, number, or underscore) or hyphen
• The email must end with at least one set of a dot followed by one or more word characters.
• The input must NOT be case-sensitive

The function should return true or false.