Regex to Convert snake_case to camelCase

snake_case to camelCase

Going the other direction, this conversion transforms identifiers like get_user_name into getUserName.

Basic Conversion (snake_case to camelCase)

str.replace(/_([a-z])/g, (_, ch) => ch.toUpperCase())

Each _x becomes X.

snake_case to PascalCase

str.replace(/(?:^|_)([a-z])/g, (_, ch) => ch.toUpperCase())

Same logic, but capitalizes the first letter too.

Tested Examples

Input	camelCase	PascalCase
foo_bar	fooBar	FooBar
get_user_name	getUserName	GetUserName
api_v2_endpoint	apiV2Endpoint	ApiV2Endpoint
_private_field	PrivateField	PrivateField
http_request	httpRequest	HttpRequest
already_lower	alreadyLower	AlreadyLower

Handle Leading Underscores Carefully

_privateField is conventional in JavaScript for private fields. To preserve a single leading underscore:

str.replace(/(_)?([a-z]+)(?:_([a-z]+))*?/, ...)

A safer approach: split on _, treat the first segment as-is or empty, and capitalize the rest.

function snakeToCamel(s) {
  if (s.startsWith("_")) {
    return "_" + snakeToCamel(s.slice(1));
  }
  return s.replace(/_([a-z0-9])/g, (_, ch) => ch.toUpperCase());
}

Double Underscores

a__b becomes a_B with the basic regex (the second underscore is preserved). To collapse:

str.replace(/_+([a-z])/g, (_, ch) => ch.toUpperCase())

From kebab-case Too

Same logic, swap the underscore for a hyphen:

str.replace(/-([a-z])/g, (_, ch) => ch.toUpperCase())

Practical Note

When mapping between API styles, enforce the convention with TypeScript types and an automatic transformation layer (e.g. humps library) rather than scattering regexes throughout the codebase.