The solution can only contain the 95 printable ASCII characters (space to ~). Tabs and newlines may be allowed as well, depending on the challenge.
These puzzles are a type restricted-source puzzles. When a puzzle is tagged with the printable-ascii tag, solutions of the puzzle should use only printable ASCII characters. (Tab and newlines may be added to increase readability.) The purpose of this tag is to prevent solutions from using compression functions to compress their code, and to prevent solutions using programming languages that utilize the wide character set of Unicode to compress their code.