/**
 * Parses a .env file string into a key-value object.
 * Supports quotes, comments, and variable expansion.
 *
 * @param input - The .env file content as a string.
 * @param expandVariables - Whether to expand variable references like $VAR or ${VAR} (default: false).
 * @returns An object containing the parsed environment variables.
 *
 * @throws {Error} If input is empty.
 * @throws {Error} If a line has invalid format.
 *
 * @example
 * // Basic usage
 * parseEnvFile("NODE_ENV=production\nPORT=3000");
 * // Returns { NODE_ENV: "production", PORT: "3000" }
 *
 * @example
 * // With quotes
 * parseEnvFile('DB_HOST="localhost"\nDB_NAME=\'mydb\'');
 * // Returns { DB_HOST: "localhost", DB_NAME: "mydb" }
 *
 * @example
 * // With variable expansion
 * parseEnvFile("HOST=localhost\nURL=http://$HOST:3000", true);
 * // Returns { HOST: "localhost", URL: "http://localhost:3000" }
 *
 * @example
 * // With comments
 * parseEnvFile("# Database config\nDB_HOST=localhost");
 * // Returns { DB_HOST: "localhost" }
 *
 * @note Comments start with `#`.
 * Quotes (single or double) around values are removed.
 * Empty lines and lines with only whitespace are ignored.
 * Variable expansion only works if expandVariables is true.
 *
 * @complexity Time: O(n), Space: O(n) - Where n is the length of input string
 */
export declare function parseEnvFile(input: string, expandVariables?: boolean): Record<string, string>;
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