Regex to Match JavaScript Arrow Functions

This regex matches JavaScript arrow function expressions. Here is the token-by-token breakdown:

(?: — Opens a non-capturing group for the parameter alternatives.

(?:[a-zA-Z_$][a-zA-Z0-9_$]*) — First alternative: matches a single parameter without parentheses. The parameter name must start with a letter, underscore, or dollar sign, followed by zero or more alphanumeric characters, underscores, or dollar signs. This matches the concise syntax like x => x * 2.

| — Alternation operator separating the two parameter forms.

([^)]*) — Second alternative: matches parenthesized parameters. The escaped parentheses match literal opening and closing parens, with zero or more non-closing-paren characters inside. This handles zero parameters (), single parameters (x), and multiple parameters (a, b, c).

) — Closes the non-capturing group.

\s* — Matches optional whitespace between the parameters and the arrow.

=> — Matches the literal arrow operator consisting of an equals sign followed by a greater-than sign. This is the defining syntax element of arrow functions.

The g flag enables global matching to find all arrow functions in the source code. Arrow functions were introduced in ES6 and provide a concise syntax for function expressions. They also lexically bind the this value, making them particularly useful in callbacks and event handlers.

This pattern is useful for code analysis, identifying modern JavaScript patterns, and migration tools that convert traditional functions to arrow syntax. It matches both expression-body and block-body arrow functions since the body follows the => which this pattern captures.