Blob Error Code & Verbatim Type code question

The spec says (for simple errors)

> The first word in the error is in upper case and describes the error code. The remaining string is the error message itself. The `ERR` error code is the generic one. The error code is useful for clients to distinguish among different error conditions without having to do pattern matching in the error message, that may change.

and (for blob errors)

> The general form is `!\r\n\r\n`. It is exactly like the String type. However like the Simple error type, the first uppercase word represents the error code.

So am I correct in assuming that the error code **must**

1. be upper-case only, or not contain anything other than `A-Z0-9-_` or some other limited character set?
2. be at the start of the error string?
3. be separated from the rest of the error by a space ` `?

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For the **verbatim** type, does the 3 character type marker have constraints? I assume it can't be `` or ``, but perhaps keeping it within the ASCII printable range?