[\w[^_]]

##</code><code>##

[     # start char class
  \w    # any word char
  [     # or a literal '['
  ^     # or a literal '^'
  _     # or an underscore (redundant...)
]     # end char class
]     # followed by a literal ']'

##</code><code>##

[^\W_]

##</code><code>##

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