Groovy
Easy: 26 characters
c={(it=~/[\W|^_]/).size()}
assert c("abcdefghijklmnopqrstuvwxyz ABCDEFGHIJKLMNOPQRSTUVWXYZ 0123456789")== 2
assert c("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789")==0
assert c("a-b-7*NNuah ~#77%")==7
assert c('_') == 1
Hard: 47 characters
s={it.replaceAll(/[\w&&[^_]]/,'').bytes.size()}
assert s("abcdefghijklmnopqrstuvwxyz ABCDEFGHIJKLMNOPQRSTUVWXYZ 0123456789")== 2
assert s("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789")==0
assert s("a-b-7*NNuah ~#77%")==7
assert s('_') == 1