tcl_nonwordchars is a global variable which contains a regular expression that is used by routines like tcl_endOfWord to identify whether a character is part of a word or not. If the pattern matches a character, the character is considered to be a non-word character. On Windows platforms, spaces, tabs, and newlines are considered non-word characters. Under Unix, everything but numbers, letters and underscores are considered non-word characters.
[add list of places within tcl and tk where this variable is used.]
See also tclvars for other variables used tcl routines