[Convert PascalCase / camelCase to snake_case]

camel_case.rb

# Convert a string from PascalCase or camelCase to snake_case
#
# @example
#   format_string_key("SearchVault") #=> "search_vault"
#   format_string_key("SetPINDirection") #=> "set_pin_direction"
#   format_string_key("PINNumber") #=> "pin_number"
#   format_string_key("myVaultPINNumber") #=> "my_vault_pin_number"
#
# @param key [String] The key name to convert
# @return [String] The converted string
def format_string_key(key)
  # e.g. key = "  MyVaultPINNumber  "
  key.strip                           #=> "MyVaultPINNumber"
     .gsub(/([a-z])([A-Z])/, '\1_\2') #=> "My_VaultPIN_Number"
     .gsub(/([A-Z][a-z])/, '_\1')     #=> "_My__Vault_PIN_Number"
     .gsub(/^_/, '')                  #=> "My__Vault_PIN_Number"
     .squeeze('_')                    #=> "My_Vault_PIN_Number"
     .downcase                        #=> "my_vault_pin_number"
end