This package implements frequent string operations: searching, replacing,
splitting, matching. It is independent from the Str library, and can
replace Str in many cases. Unlike Str, xstr is thread-safe. xstr does
not implement regular expressions in general, but an important subset.
Some operations of xstr are performed as quickly as by Str; if the string
to be processed is small, xstr is often faster than Str; if the string is
big, xstr is up to half as fast than Str.

WWW: http://projects.camlcity.org/projects/xstr.html