Tcl/Tk 8.1 features Unicode support for internationalization, thread safety, and a new regular expression package.
Tcl 8.1 should probably have been called Tcl 9.0. The internal changes required to support Unicode caused a major overhaul that touched nearly the entire implementation. At the same time, the code base was cleaned up so that it could be used in multithreaded environments, and it added a platform-independent dynamic loading facility (i.e., stub libraries). Finally, thanks to Henry Spencer, an all new regular expression package was added that brings Advanced Regular Expressions to Tcl. However, in spite of all these changes, scripts written for earlier versions of Tcl are very compatible with Tcl 8.1.