Syntax Rules (Cambridge (CIE) A Level Computer Science): Revision Note

BNF & syntax diagrams

• When writing programs, you must follow a set of strict rules known as syntax — the structure that defines how code must be written.

  • Just like spoken languages have grammar rules, programming languages have syntax rules

  • Each language has its own syntax, e.g. Python uses indentation, Java uses semicolons

• Programs are first written in source code (a high-level language like Python, Java, etc.)

• To run the program, this source code must be translated into machine code

• For translation to work, syntax must be precisely defined to avoid ambiguity

• Translators (compilers or interpreters) rely on these rules to correctly convert the program into a form the CPU can understand

Regular expressions and meta-languages

• Regular expressions are used to describe simple patterns in text or syntax

• Many programming constructs (like variable names or simple statements) can be defined using regular expressions

• However

  • Some aspects of a programming language are too complex for regular expressions alone

• When regular expressions aren't enough, we use a meta-language

• A language used to describe the structure of another language

• The most common example is Backus-Naur Form (BNF) or syntax diagrams

• These allow us to define:

  • Full grammar rules

  • Nested structures

  • Hierarchical syntax

What is Backus-Naur form (BNF)?

• BNF is a meta-language, a way of writing rules that define the syntax of programming languages or data structures

• BNF is used to:

  • Describe formal grammar

  • Define how statements and expressions are structured

  • Ensure that syntax rules are unambiguous and machine-readable

• BNF uses a set of rules made up of:

Example 1

<digit> ::= 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9
<number> ::= <digit> | <digit> <number>

• <digit> is defined as any single digit 0–9

• <number> is defined as:

  • a single digit, or

  • a digit followed by another number (i.e. multiple digits)

Example 2

• <number> contains one or more digits

• BNF for <number> and use it to define <IDcode>, which consists of a <number> followed by either a <letter> or another <number>

<number> ::= <digit> | <number><digit>
<IDcode> ::= <number><letter> | <number><number>

• Recursive definition of one or more items

• Use of | for choice

• Application to a second rule with multiple components

What is a syntax diagram?

• A syntax diagram is a graphical representation of BNF (meta-language)

• It uses symbols to map directly to the structure of BNF

• The symbols used are:

Examples