ruby on rails - How to validate that username does not start with certain string? -

one of variables in user model username. reserve usernames starting "vondel" else. therefore, make invalid username start "vondel". how can this?

below attempt set validations on username. however, generating username starts "vondel" still possible , generates no error message. there way regular expression?

before_save :downcase_username        # method converts username downcase. valid_username_regex = /\a[a-za-z0-9_-]+\z/i validates :username, presence: true,                      length: { in: 6..15 },                      format: { with: valid_username_regex },                      uniqueness: { case_sensitive: false } validate :vondel_not_allowed  private    def vondel_not_allowed     if username == "vondel*"       errors.add(:username, "username cannot start 'vondel'")     end   end 

the straightforward way use starts_with? method:

test_string = "vondelfoo" test_string.starts_with?("vondel") # => true 

just aside, recommend learn how regex. regexes invaluable tool, , should learn how use them.

also tip - i've heard rubular tool getting started ruby regular expressions.

edit

the tin man brings point - regexps powerful overly complex solution problem. although highly recommend learn how use them, should prefer straightforward , simple solution here.