When is a problem too complex for a regular expression?

(())()

b[A-Z0-9._%+-]+@[A-Z0-9.-]+.[A-Z]{2,4}b

(?:[a-z0-9!#$%&'*+/=?^_`{|}~-]+(?:.[a-z0-9!#$%&'*+/=?^_`{|}~-]+)*|"
(?:[x01-x08x0bx0cx0e-x1fx21x23-x5bx5d-x7f]|\[x01-x09x0bx
0cx0e-x7f])*")@(?:(?:[a-z0-9](?:[a-z0-9-]*[a-z0-9])?.)+[a-z0-9]
(?:[a-z0-9-]*[a-z0-9])?|[(?:(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?).)
{3}(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?|[a-z0-9-]*[a-z0-9]:(?:[x01-x08
x0bx0cx0e-x1fx21-x5ax53-x7f]|\[x01-x09x0bx0cx0e-x7f])+)])