casefolding

Case folding is a text processing technique used to enable case-insensitive comparisons across Unicode text. It transforms strings according to the Unicode case folding rules rather than simple lowercasing, producing a form suitable for matching. Unlike ordinary lowercasing, case folding is language-neutral and may produce multi-character mappings. For example, the German character ß (eszett) maps to "ss" in case folding, and the string "Straße" case-folds to "strasse". Some characters may map to different lengths or sequences, and not all language-specific distinctions are preserved. The aim is to provide caseless equality across scripts and alphabets rather than to generate user-visible text.

This operation is defined by the Unicode Standard, notably Unicode Technical Report #36 and the casefolding data. It is widely used in search indexing, database querying, and other algorithms where case-insensitive matching is required across multiple languages and scripts.

In practice, many programming environments offer a case-folding function or method; for instance, Python provides str.casefold() to perform Unicode-aware case folding. Implementations may differ in edge cases, and case folding does not replace locale-aware lowercasing for display or sorting. It should be chosen when the task is to determine whether two strings are equivalent under case-insensitive comparison rather than to show normalized text.