U000D

U+000D, commonly written as U000D in some contexts, is a Unicode code point that represents the carriage return (CR) control character. It is the C0 control character with the decimal value 13 and is categorized in Unicode as a non-printing control character.

Function and history: Carriage return originated with traditional typewriters, where pressing the return lever would move the print head back to the start of the line without advancing to a new line. In modern computing, CR is used as a control instruction rather than a printable symbol. It is frequently paired with the line feed character (U+000A) to form a newline sequence in many systems.

Encoding and representations: In UTF-8, U+000D is encoded as a single byte 0x0D. In UTF-16, it appears as 0x000D. As a non-printing character, it does not have a visible glyph. In strings and regular expressions, it is often represented as \r.

Usage in systems and protocols: Different operating systems adopt different newline conventions. Windows typically uses CR followed by LF (CRLF) to terminate lines, while Unix-like systems use LF alone, and classic Mac OS used CR alone. Certain network protocols, such as HTTP and SMTP, specify CRLF as the standard end-of-line terminator. CR can also be used on its own in legacy contexts, though this is now uncommon.

Impact on text handling: Because CR is a control character, it can affect text rendering, file parsing, and data interchange if not interpreted correctly. Software may normalize line endings or treat CR as whitespace in some contexts, leading to compatibility issues if mixed conventions are encountered.