Regex to Match JavaScript Function Declarations

This regex matches JavaScript function declarations in the standard syntax. Here is the token-by-token breakdown:

function — Matches the literal keyword function that begins a function declaration.

\s+ — Matches one or more whitespace characters between the keyword and the function name.

([a-zA-Z_$][a-zA-Z0-9_$]*) — Capturing group 1 matches the function name. JavaScript identifiers must start with a letter, underscore, or dollar sign, followed by zero or more letters, digits, underscores, or dollar signs. This captures names like myFunction, _private, $helper, and calculateTotal.

\s* — Matches optional whitespace between the function name and the opening parenthesis.

( — Matches the literal opening parenthesis that begins the parameter list. The parenthesis is escaped because it is a regex metacharacter used for grouping.

[^)]* — Matches zero or more characters that are not a closing parenthesis. This captures the entire parameter list including parameter names, default values, destructuring patterns, and commas between parameters.

) — Matches the literal closing parenthesis that ends the parameter list.

The g flag enables global matching to find all function declarations in the source code. This pattern is useful for code analysis, function indexing, documentation generation, and refactoring tools. It matches standard function declarations but not arrow functions, function expressions assigned to variables, or class methods. For those patterns, separate regex patterns are needed.