Identifiers are used both as names for VHDL objects, procedures, functions, processes, design entities, etc., and as reserved words. There are two classes of identifiers: basic identifiers and extended identifiers.

The basic identifiers are used for naming all named entities in VHDL. They can be of any length, provided that the whole identifier is written in one line of code. Reserved words cannot be used as basic identifiers (see reserved words for complete list of reserved words). Underscores are significant characters in an identifier and basic identifiers may contain underscores, but it is not allowed to place an underscore as a first or last character of an identifier. Moreover, two underscores side by side are not allowed as well. Underscores are significant characters in an identifier.

The extended identifiers were included in VHDL '93 in order to make the code more compatible with tools which make use of extended identifiers. The extended identifiers are braced between two backslash characters. They may contain any graphic character (including spaces and non-ASCII characters), as well as reserved words. If a backslash is to be used as one of the graphic characters of an extended literal, it must be doubled. Upper- and lower-case letters are distinguished in extended literals.

Examples

Example 1

-- legal identifiers
Decoder_1 FFT Sig_N Not_Ack
\signal\ \C:\\Cads\ \Signal @#\

All above identifiers are legal in VHDL '93. Note that a single backslash inside an extended name is denoted by two backslashes.

Example 2

-- illegal identifiers
_Decoder_1 2FFT Sig_#N Not-Ack

No VHDL tool would accept any of the four identifiers above. The errors are as follows: underscore at the beginning of the first identifier, digit at the beginning of the second, special character (#) inside the third and hyphen (special character as well) in the fourth.

Important Notes

A basic identifier must begin with a letter.
No spaces are allowed in basic identifiers.
Basic identifiers are not case sensitive, i.e. upper- and lower-case letters are considered identical.
Basic identifiers consist of Latin letters (a..z), underscores ( _ ) and digits (0..9). It is not allowed to use any special characters here, including non-Latin (language-specific) letters.