What should I call a highly restricted username field?

I am designing an account creation form that will prompt for username and pass phrase. The username will represent the user for login and communication with other users. It will also be used across multiple protocols (e.g. email prefix), including ones not yet planned for, so I need to severely restrict input to values that will be compatible with multiple protocols (e.g. no periods). The restrictions are as follows:

  • must begin a-z
  • can be followed by a sequence of characters from a-z and/or 0-9
  • must be lowercase
  • nothing else is permitted (uppercase, punctuation, unicode...)

So a number of typical workarounds for usernames are not permitted: jane.doe, JohnDoe and Björk_Guðmundsdóttir would all be invalid.

What could I call this field so that it still feels like the user's identity, without disappointing them (too much) when they realise the restrictions?

(I thought of having a separate "full name" or "what shall we call you" field but I can't guarantee that I'll use that functionality.)

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