The syntax of computer source code is code structured and ordered restricted to computer language rules. Like a natural language, a computer language (i.e. a programming language) defines the syntax that is valid for that language. A syntax error occurs when syntactically invalid source code is processed by an tool such as a compiler or interpreter.

The most commonly used languages are text-based with syntax based on strings. Alternatively, the syntax of a visual programming language is based on relationships between graphical elements.

When designing syntax, a designer of any language might start by writing down examples of both legal and illegal strings, before trying to figure out the general rules from those examples in use. Such that its general structure of syntax can be determined through its form of composition, so to yield semantically valid ranges everytime for each possible alteration. Or else, it returns errors, and warnings, for each invalid input.

Computer language syntax is generally distinguished into three levels:

Distinguishing in this way yields modularity, allowing each level to be described and processed separately and often independently.

First, a lexer turns the linear sequence of characters into a linear sequence of tokens; this is known as "lexical analysis" or "lexing".

Second, the parser turns the linear sequence of tokens into a hierarchical syntax tree; this is known as "parsing" narrowly speaking. This ensures that the line of tokens conform to the formal grammars of the programming language. The parsing stage itself can be divided into two parts: the parse tree, or "concrete syntax tree", which is determined by the grammar, but is generally far too detailed for practical use, and the abstract syntax tree (AST), which simplifies this into a usable form. The AST and contextual analysis steps can be considered a form of semantic analysis, as they are adding meaning and interpretation to the syntax, or alternatively as informal, manual implementations of syntactical rules that would be difficult or awkward to describe or implement formally.

Thirdly, the contextual analysis resolves names and checks types. This modularity is sometimes possible, but in many real-world languages an earlier step depends on a later step – for example, the lexer hack in C is because tokenization depends on context. Even in these cases, syntactical analysis is often seen as approximating this ideal model.