R:
gsub("|[][!#$%()?*,.:;&<=>@^_`|~.{}/\\\\]","\\1","hi! this* (i[s a~ special? charac^ter* \\removal/ line #of &c%o$d@e",fixed = F)
gsub(" |[][!#$%()?*,.:;<=>@^_`|~.{}/\\\\]","\\1","I(alsoremovesspaces)",fixed = F)


bash:

sed:
echo "hi! this* (i[s a~ special? charac^ter* \\re[m]oval/ line #of &c%od@e" | sed 's/[]\\[!#%(")?*,.:;&<=>@^_`~.{}\\\\\/]//g'