Issue #20438 has been updated by nobu (Nobuyoshi Nakada). https://github.com/ruby/ruby/pull/10857 ---------------------------------------- Bug #20438: String format "%\n" and "%\0" does not raise format error https://bugs.ruby-lang.org/issues/20438#change-108461 * Author: tompng (tomoya ishida) * Status: Open * ruby -v: ruby 3.3.0 (2023-12-25 revision 5124f9ac75) +YJIT +MN [arm64-darwin22] * Backport: 3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN ---------------------------------------- `"%" % 1` raises `incomplete format specifier; use %% (double %) instead` `"%=" % 1` raises `malformed format string - %=`. But `"%\n" % 1` `"%\0" % 1` does not raise error. In `sprintf.c`, `\n` and `\0` are explicitly accepted. Is this expected? Some other language are: Perl: Warns `Invalid conversion in printf`. Just prints. `"%d% " → "1% "` Python: Error `ValueError: unsupported format character '?' (0xa)` with `print("%\n" % 123)` PHP: Error `Unknown format specifier` with `sprintf("%\n", 3)` C, C++: Warns `incomplete format specifier`. `"%\n" → "\n"`, `"%" → ""`, `"% " → ""` `sprintf("%f%\n",x)` and `"%f%\n" % x` is used in some codes https://github.com/search?q=language%3ARuby+%22f%25%5Cn%22&type=code -- https://bugs.ruby-lang.org/