\W is the opposite. It matches any non-word character and is equivalent to [^a-zA-Z0-9_]:

[imsanjoykb]

re.search('\W', 'a_13Qb')
<_sre.SRE_Match object; span=(3, 4), match=''>
re.search('[^a-zA-Z0-9_]', 'a_13Qb')
<_sre.SRE_Match object; span=(3, 4), match=''>