Basic identifiers consist of letters, underscores and digits and must start with a letter. They are not case sensitive. Reserved words of the language cannot be basic identifiers. Examples of valid VHDL basic identifiers:
A_myId90 a_MYID90 abcDEf100_1 ABCdef100_1
The two first are equivalent and the two last are also equivalent (case insensitivity).
Examples of invalid basic identifiers:
_not_reset -- start with underscore 85MHz_clock -- start with digit LooP -- reserved word of the language