tcl_wordchars 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 word character. On Windows platforms, words are comprised of any character that is not a space, tab, or newline. Under Unix, words are comprised of numbers, letters or underscores.
See tclvars for other variables used by tcl/tk.