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in reply to Re^2: One regex construct to handle multiple string types
in thread One regex construct to handle multiple string types

Precise definition depends on the language.

Mastering Regular Expressions, 2nd Ed., Jeffery E. F. Friedll, published by O'Reilly characterizes\w in its "Common Metacharacters..." chapter, this way:

Part-of_word character   Often the same as [a-zA-Z0-9_], although some ools omit the underscore, while others include all the extraalphanumerics characters in the locale. If Unicode is supported, \w usually refers to all alphanumerics (notable exception: Sun's Java regex package whose \w is exactly [a-zA-Z0-9_</c>).
Regular Expressions Pocket Reference (also from O'Reilly) defines \w as:

Regretably, the definition of \p{isWord} -- [_\p{L1}\p{Lu}\p{Lt}\p{Lo}\p{Nd} -- is, for me, almost impenetrable but Friedll's characterization may be as good as you'll get without deep study of perlretut and friends.