one way is to use tr to remove unwanted character types. -dc removes everything except those char types you specify. [:alnum;] is alphanumeric, so if you check the length of the string it should be unchanged if it contains only the char types you want.
Code:
# t is the input string, ck is a variable to check the contents of t
t="thisisa555ctest"
ck=$( echo "$t" | tr -dc '[:alnum:]')
if [[ ${#t} -eq ${#ck} ]]; then
echo "ok"
else
echo "not ok"
exit 1
fi
# t is all good chars at this point
# check length of t
if [[ ${#t} -gt 50 ]]; then
echo "not ok too long"
fi
