[\w[^_]]
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Nested character classes aren't implemented yet... That will parse somthing like this:

[     # start char class
  \w    # any word char
  [     # or a literal '['
  ^     # or a literal '^'
  _     # or an underscore (redundant...)
]     # end char class
]     # followed by a literal ']'
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If you want a character class consisting of all the word chars except underscore, you need to use the double negative (and somewhat non-intuitive):

[^\W_]
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Which matches a character that is not a non word char (i.e a word char) and not an underscore.

% perl -le '/[^\W_]/ && print for qw(a b _ c d)'
a
b
c
d
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-Blake