This command is part of the textutil package in tcllib. It splits a string up according to a regular expression, defaulting to splitting by any (non-empty) sequence of whitespace characters.

textutil::splitx string ?regexp?

This splits string into a list of words, where regexp is a regular expression that matches the parts of string that separate the words. The result is a proper Tcl list. If regexp is omitted, it defaults to a regular expression that matches any non-empty sequence of whitespace characters.