What are '$$' used for in PL/pgSQL
问题 Being completely new to PL/pgSQL , what is the meaning of double dollar signs in this function: CREATE OR REPLACE FUNCTION check_phone_number(text) RETURNS boolean AS $$ BEGIN IF NOT $1 ~ e\'^\\\\+\\\\d{3}\\\\ \\\\d{3} \\\\d{3} \\\\d{3}$\' THEN RAISE EXCEPTION \'Wrong formated string \"%\". Expected format is +999 999\'; END IF; RETURN true; END; $$ LANGUAGE plpgsql STRICT IMMUTABLE; I\'m guessing that, in RETURNS boolean AS $$ , $$ is a placeholder. The last line is a bit of a mystery: $$