BNF

A context-free language is a set of strings and symbols that follow the rules of a context-free grammar.

A CFG describes which strings are possible or not possible in a language by defining a set of production rules.

A production rule specifies how one symbol can be replaced by another. Examples: a → ab (a is replaced by ab) a → aa (a is replaced by aa) b → a (b is replaced by a)

BNF is a way of notating context-free languages using statements where the left-hand side is defined by the right-hand side.

Non-terminals are syntactic variables placed inside angle brackets (e.g. <expr>). They can be broken down further into terminals, other non-terminals, or both.</expr>

Terminals are literal symbols (without brackets) that cannot be broken down further and must be taken as written values.

It represents the OR operator, allowing alternatives in production rules.

A non-terminal can be defined in terms of itself, allowing recursive definitions. This enables BNF to represent some strings that regular expressions cannot, because regex does not support recursion.

It allows representation of nested or repeating structures (e.g. balanced parentheses, nested expressions) which cannot be represented by regular expressions.

A syntax diagram is a visual representation of a regular language. Non-terminals are shown as rectangles. Terminals are shown as ellipses. Arrows connect shapes to show how strings can be formed.

Each non-terminal has its own separate diagram that defines its production rules visually.