casefolds

Casefolds refer to the process of converting text to a form suitable for case-insensitive comparison by applying Unicode case folding rules. Case folding is designed to be language- and locale-independent, enabling consistent caseless matching across scripts and alphabets. It is defined in the Unicode Standard as a form of mapping from code points to a caseless representation, and includes not only simple lowercase mappings but also expansions where a single character becomes multiple letters, as well as ligature expansions.

In practice, casefolding often differs from simple lowercasing. For example, the German character ß lowercases to “ß” but casefolding produces “ss” (and uppercase ß is seldom used); ligature characters such as “fi” may be expanded to “fi” or “ffi” depending on the mapping. In general, casefold aims to maximize the odds that two strings compare equal under a case-insensitive comparison.

Implementation and usage: Many programming environments provide a casefold function or method. In Python, strings expose casefold(): “ß”.casefold() -> “ss”. In ICU-based libraries, there is a CaseFolding operation derived from Unicode. Case folding is intended for comparisons, not for display or normalization of text; it is not a locale-aware transformation.

Relationship to normalization: Case folding is distinct from Unicode normalization; both may be applied, but for comparison purposes, case folding should precede or accompany normalization depending on the pipeline. See Unicode Standard Annex 21 for the exact mappings.