I don't think it can - the local-part of the original matches as (a letter followed by letters) or (a letter followed by numbers):

    [A-Za-z]([A-Za-z]+|(?:\d+))
    =>
    [A-Za-z]([A-Za-z]+|\d+)

Your version doesn't match this: - it allows numbers and digits to be interleaved - it allows the local-part to start with a digit

    [a-z0-9]+ != ([a-z]+|\d+)