specialcasing

Specialcasing refers to a set of rules for transforming the case of letters that depend on language, locale, or context, rather than applying a single, universal mapping. In Unicode and related text processing systems, special casing handles exceptions to the general uppercase-lowercase conversions and is defined in data files such as SpecialCasing.txt. These rules are used when proper orthography and phonology require locale-aware transformations.

Common examples include Turkish and Azerbaijani: the uppercase of the letter i is İ (I with a dot) and the lowercase of I is ı (dotless i). Because of this, simple, language-agnostic case conversion can produce incorrect results in these languages, so special casing provides the correct mappings for each locale. Another well-known case concerns German, where the lowercase ß maps to SS when uppercased, and, in some contexts, a distinct capital form ẞ (capital sharp S) is recognized in Unicode. Greek also has context-dependent rules for the letter sigma: the lowercase form σ is used inside words, while final-position forms ς appear at the end of words, with corresponding uppercase mappings to Σ.

Applications of special casing include correct display in typography, accurate search and indexing, and locale-aware sorting and comparison. It complements general case folding and locale-sensitive operations in programming languages and libraries (such as Unicode, ICU, and language‑specific runtimes) to ensure linguistically correct and visually appropriate text transformations. Limitations arise from the diversity of languages and evolving orthographic standards, so implementations must reference up-to-date data like the Unicode SpecialCasing rules.