Definition
Case Folding
Case folding maps strings to common forms for case-insensitive matching. Under a case-folding function
, strings match without regard to letter case when
Unicode Case Folding
For basic Latin letters, folding agrees with lowercasing. Full Unicode case folding is not merely lowercasing: it can also expand characters, such as mapping ß to ss.
Retrieval Trade-off
Capitalisation can distinguish names or acronyms from ordinary words, as in Fed versus fed and SAIL versus sail. Folding loses this distinction, but allows matching when users omit the expected capitals. Whether to retain case distinctions depends on the retrieval task.
Example
Letter case
Beyond lowercasing
Full Unicode case folding can match the following German spellings:
Simply lowercasing both inputs would leave straße and strasse distinct.