NFKC

Normalization Form Compatibility Composition, abbreviated NFKC, is a Unicode normalization form used to transform text into a form that is stable for comparison, indexing, and display. It belongs to the family of Unicode normalization forms that also includes NFC, NFD, and NFKD.

NFKC works in two steps. First, compatibility decomposition replaces each character with one or more basic characters according to compatibility mappings. Then, canonical composition is applied to the resulting sequence to reassemble characters where possible into precomposed forms. The result is a string that is canonically composed but may differ in its underlying code points from the original.

NFKC is more aggressive than NFC and NFD because it handles compatibility characters and formatting distinctions. Compared with NFKD, it not only decomposes but also recomposes, producing a text form that may be visually equivalent to the original in many contexts but with a different internal representation. This makes NFKC suitable for comparing text where compatibility differences should be ignored, such as certain search or indexing tasks.

Examples illustrate its behavior. The ligature “ff” (U+FB00) is decomposed to the two letters “ff” under NFKC. The compatibility character for a Roman numeral, such as U+2160 (Ⅰ), is mapped to the simple letter “I” in NFKC.

Cautions include that NFKC can alter semantic content or identifiers and may enable homoglyph-based attacks in security contexts. It is widely supported in programming libraries and is typically exposed as a normalization option (e.g., normalize or normalizeNFKC). Related forms are NFC, NFD, and NFKD.