Great, I thought this feature would be more complex to implement.
There's an issue though, it is possible in common to register usernames starting with a whitespace.
For example ( whitespaceusername), which would result in just colored circle then. Another variants are some non-letter symbols being 1st, like -, +, _, [, and ], but those are visible at least :)

Great, I thought this feature would be more complex to implement. There's an issue though, it is possible in common to register usernames starting with a whitespace. For example ( whitespaceusername), which would result in just colored circle then. Another variants are some non-letter symbols being 1st, like -, +, _, [, and ], but those are visible at least :)

that makes me think we should not allow usernames to start with a white space?

we should not allow usernames to start with a white space

Easier way:

{{ USERNAME|striptags|trim(' -+_[]', 'left')|slice(0, 1)|upper }}