VHDL:identifier

Identifiers are used to name items in a VHDL.

The rules to form identifiers are:

• use only: alphabetic letters (‘A’ to ‘Z’ and ‘a’ to ‘z’), decimal digits (‘0’ to ‘9’) and underline character (‘_’);
• start with an alphabetic letter (‘A’ to ‘Z’ and ‘a’ to ‘z’);
• do not end with '_';
• do not use sucessive '__'.