How to build a method to validate emails
def email_is_junk(email_address)
end

USER_RULES = ['+', 'do-not-reply', 'support', 'test', 'service', 'tips', 'twitter', 'alerts', 'survey']
DOMAIN_RULES = ['craigslist.org']

def email_is_junk(email)
  return true if !email.match('@') # return early if no @
  user, domain = email.split('@')
  USER_RULES.each   { |rule| return true if user.match(rule)   }
  DOMAIN_RULES.each { |rule| return true if domain.match(rule) }
  false # reached the end without matching anything
end

Regexp.union                         #=> /(?!)/
Regexp.union("penzance")             #=> /penzance/
Regexp.union("a+b*c")                #=> /a+b*c/
Regexp.union("skiing", "sledding")   #=> /skiing|sledding/
Regexp.union(["skiing", "sledding"]) #=> /skiing|sledding/
Regexp.union(/dogs/, /cats/i)        #=> /(?-mix:dogs)|(?i-mx:cats)/

Regexp.escape('*?{}.')   #=> \*?{}.

patterns = [
  /.+?+.+?@/
]

strings = [
    'do-not-reply', 'support', 'test', 'service', 'tips', 'twitter', 'alerts', 'survey',
    'craigslist.org'
]

regex = Regexp.union(
  *patterns,
  *strings.map{ |s|
    Regexp.new( Regexp.escape("#{ s }@"), Regexp::IGNORECASE ) }
)
pp regex

>> /(?-mix:.+?+.+?@)|(?i-mx:do-not-reply@)|(?i-mx:support@)|(?i-mx:test@)|(?i-mx:service@)|(?i-mx:tips@)|(?i-mx:twitter@)|(?i-mx:alerts@)|(?i-mx:survey@)|(?i-mx

sample_email_addresses = %w[
    user
    user+foo
    do-not-reply
    support
    service
    tips
    twitter
    alerts
    survey
].map{ |e| e << '@host.com' }

pp sample_email_addresses.map{ |e| [e, !!e[regex]] }

>> [["[email protected]", false],
>> ["[email protected]", true],
>> ["[email protected]", true],
>> ["[email protected]", true],
>> ["[email protected]", true],
>> ["[email protected]", true],
>> ["[email protected]", true],
>> ["[email protected]", true],
>> ["[email protected]", true]]

pp sample_email_addresses.select{ |e| e[regex] }

>> ["[email protected]",
>>  "[email protected]",
>>  "[email protected]",
>>  "[email protected]",
>>  "[email protected]",
>>  "[email protected]",
>>  "[email protected]",
>>  "[email protected]"]

pp sample_email_addresses.reject{ |e| e[regex] }

>> ["[email protected]"]

function isJunk(email) {
  return hasPlus(email) || supportLike(email) || craigsList(email);
}

function craigsList(email) {
  return email.match(/@craigslist.org/);
}

function supportLike(email) {
  return email.match(/do-not-reply|support|test|service|tips|twitter|alerts|survey/);
}

function hasPlus(email) {
  return email.match(/+.*@/);
}