Docs for IS_EMAIL

    Checks if field's value is a valid email address. Can be set to disallow
    or force addresses from certain domain(s).

    Email regex adapted from
    http://haacked.com/archive/2007/08/21/i-knew-how-to-validate-an-email-address-until-i.aspx,
    generally following the RFCs, except that we disallow quoted strings
    and permit underscores and leading numerics in subdomain labels

    Args:
        banned: regex text for disallowed address domains
        forced: regex text for required address domains

    Both arguments can also be custom objects with a match(value) method.

    Example:
        Check for valid email address::

            INPUT(_type='text', _name='name',
                requires=IS_EMAIL())

        Check for valid email address that can't be from a .com domain::

            INPUT(_type='text', _name='name',
                requires=IS_EMAIL(banned='^.*\.com(|\..*)$'))

        Check for valid email address that must be from a .edu domain::

            INPUT(_type='text', _name='name',
                requires=IS_EMAIL(forced='^.*\.edu(|\..*)$'))

            >>> IS_EMAIL()('a@b.com')
            ('a@b.com', None)
            >>> IS_EMAIL()('abc@def.com')
            ('abc@def.com', None)
            >>> IS_EMAIL()('abc@3def.com')
            ('abc@3def.com', None)
            >>> IS_EMAIL()('abc@def.us')
            ('abc@def.us', None)
            >>> IS_EMAIL()('abc@d_-f.us')
            ('abc@d_-f.us', None)
            >>> IS_EMAIL()('@def.com')           # missing name
            ('@def.com', 'enter a valid email address')
            >>> IS_EMAIL()('"abc@def".com')      # quoted name
            ('"abc@def".com', 'enter a valid email address')
            >>> IS_EMAIL()('abc+def.com')        # no @
            ('abc+def.com', 'enter a valid email address')
            >>> IS_EMAIL()('abc@def.x')          # one-char TLD
            ('abc@def.x', 'enter a valid email address')
            >>> IS_EMAIL()('abc@def.12')         # numeric TLD
            ('abc@def.12', 'enter a valid email address')
            >>> IS_EMAIL()('abc@def..com')       # double-dot in domain
            ('abc@def..com', 'enter a valid email address')
            >>> IS_EMAIL()('abc@.def.com')       # dot starts domain
            ('abc@.def.com', 'enter a valid email address')
            >>> IS_EMAIL()('abc@def.c_m')        # underscore in TLD
            ('abc@def.c_m', 'enter a valid email address')
            >>> IS_EMAIL()('NotAnEmail')         # missing @
            ('NotAnEmail', 'enter a valid email address')
            >>> IS_EMAIL()('abc@NotAnEmail')     # missing TLD
            ('abc@NotAnEmail', 'enter a valid email address')
            >>> IS_EMAIL()('customer/department@example.com')
            ('customer/department@example.com', None)
            >>> IS_EMAIL()('$A12345@example.com')
            ('$A12345@example.com', None)
            >>> IS_EMAIL()('!def!xyz%abc@example.com')
            ('!def!xyz%abc@example.com', None)
            >>> IS_EMAIL()('_Yosemite.Sam@example.com')
            ('_Yosemite.Sam@example.com', None)
            >>> IS_EMAIL()('~@example.com')
            ('~@example.com', None)
            >>> IS_EMAIL()('.wooly@example.com')       # dot starts name
            ('.wooly@example.com', 'enter a valid email address')
            >>> IS_EMAIL()('wo..oly@example.com')      # adjacent dots in name
            ('wo..oly@example.com', 'enter a valid email address')
            >>> IS_EMAIL()('pootietang.@example.com')  # dot ends name
            ('pootietang.@example.com', 'enter a valid email address')
            >>> IS_EMAIL()('.@example.com')            # name is bare dot
            ('.@example.com', 'enter a valid email address')
            >>> IS_EMAIL()('Ima.Fool@example.com')
            ('Ima.Fool@example.com', None)
            >>> IS_EMAIL()('Ima Fool@example.com')     # space in name
            ('Ima Fool@example.com', 'enter a valid email address')
            >>> IS_EMAIL()('localguy@localhost')       # localhost as domain
            ('localguy@localhost', None)