SurrogatePaare

SurrogatePaare, commonly called surrogate pairs, are a mechanism used in UTF-16 encoding to represent code points outside the Basic Multilingual Plane (BMP), i.e., code points from U+10000 to U+10FFFF. In UTF-16, such a code point is encoded as two 16‑bit code units: a high surrogate and a low surrogate. The high surrogate range is 0xD800 to 0xDBFF, and the low surrogate range is 0xDC00 to 0xDFFF. The numeric value of a surrogate pair is computed as U = ((H - 0xD800) << 10) + (L - 0xDC00) + 0x10000, where H and L are the high and low surrogate values.

Example: U+1F600 (GRINNING FACE) is encoded as the pair 0xD83D 0xDE00. In memory this pairing may appear as the corresponding bytes, with endianness determining the exact byte order (for instance, UTF-16BE stores the 16‑bit units as D8 3D and DE 00).

Endianness and well-formedness are important: the two code units form a single character only when used together as a surrogate pair. Unpaired surrogates (a high surrogate not followed by a low surrogate, or a low surrogate without a preceding high surrogate) are not valid in well-formed UTF-16 and are typically handled by decoders through replacement or error signaling.

Usage and implications: many programming languages expose strings as sequences of code units or code points, which affects length, indexing, and iteration. Proper handling of surrogate pairs is essential for correctly processing characters outside the BMP and for interoperating with UTF-16-based data.

See also: Unicode, UTF-16, surrogate half, supplementary plane, code point, UTF-8.