Custom fields allows you to store additional subscriber information with a mobile phone number. You can then use these custom fields to collect subscriber information during the opt-in process, use them as merge tags in mass messages, and even use them to segment messages to certain subscribers.
For more information on custom fields, see the video below.
In the video, we use the regular expression: \A(happy|sad)\z, which accepts both the words "happy" and "sad" and rejects everything else. The regular expression, whether it is accepted or denied, is not case sensitive. So for this example, Tatango will accept happy, Happy, HAPPY, etc.
To test regular expression, you can use http://www.rubular.com/. When testing, be sure to put a "i" after "/", as this will make your test not case sensitive, which is how Tatango works.