casefold

Casefold is a process used to convert text into a form that can be compared in a case-insensitive way. It is based on Unicode case folding and is intended for caseless matching across different scripts and languages. Case folding is more expansive than simple lowercasing, because it may map certain characters to multiple code points or expand ligatures into their component letters.

In programming, casefolding is typically exposed as a function or method that returns the folded form of a string. It is designed for robust text comparisons and hashing, where language-agnostic matching is important. Casefold differs from lowercase in that it uses a broader Unicode folding mapping; for example, the German letter ß is folded to "ss," and certain ligatures may be expanded to multiple characters.

Examples illustrate its behavior but are not meant to preserve the original appearance of text. For instance, the string containing the German ß may become "ss" after casefolding, and ligatures such as some typographic forms can be expanded to their constituent letters. The result is intended to enable reliable caseless comparisons across languages and scripts rather than to produce a visually canonical form.

Applications include search, matching, and normalization tasks where case should be ignored. Casefolding is designed for stable, language-agnostic comparisons, though it is not reversible in general. It is defined in Unicode as part of the Unicode Case Folding specification and is implemented in languages and libraries that support Unicode-aware text processing, including Python’s str.casefold().

See also: Unicode Case Folding, Unicode Standard, Unicode Technical Standard UAX 21.