From Apache OpenOffice Wiki
Module regexp contains regular expression matching, some older glibc code enhanced with Unicode support and casefolding from module i18nutil and uses the Transliteration service. Completely modularized to meet the LGPL requirements. It is used by i18npool's implementation of the TextSearch service. Please do not use directly in other code, use the service instead.
On a long-term objective this should be replaced with functionality from the ICU regexp engine.