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.

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[[add list of places within [tcl] and [tk] where this variable is used.]]

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See also [tclvars] for other variables used tcl routines 

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[Category Internals]