VHDL's syntax is derived from ADA. It is strongly typed and case insensitive.
An identifier in VHDL must begin with a letter and can be any combination of letters, digits, and underscore (_).
A comment in VHDL is denoted with a "--":
-- Assign the current value to the next state value sQ_next <= sQ;
Everything after the "--" to the end of the line is considered a comment.
-- This is a valid comment sQ_next <= sQ; -- This is also a valid comment
The following words are VHDL keywords and cannot be used for identifiers (signal names, process names, entity names, etc...)
This is an incomplete list...
and, or, nor, xor, not, architecture, entity, is, process, procedure, function, type, subtype, array, begin, end, if, elsif, end, case, when, others, configuration, package, constant, signal, variable, component, label, port, generic, all, nand, nor, abs, generate, in, out, inout, buffer, linkage, bus, library, null, loop, for, body, to, downto
VHDL '87 keywords and:
group, impure, inertial, literal, postponed, pure, reject, rol, ror, shared, sla, sll, sra, srl, unaffected, xnor
VHDL '93 and '87 keywords, additionally:
generate map, access, mod, severity, units, after, until, alias, guarded, use, new, disconnect, next, protected, attribute, record, of, register, on, block, exit, open, rem, transport, report, file, return