is a number with shell

input=(73 foobar)
rg="^[+\-]{0,1}[0-9]+$"

for num in "${input[@]}"; do
    (echo "${num//[^\-+0-9]/}" | grep -Eq $rg) \
        && echo "$num is a number" \
        || echo "$num is not a number";

    nnum="-"$num
    (echo "${nnum//[^\-+0-9]/}" | grep -Eq $rg) \
        && echo "$nnum is a number" \
        || echo "$nnum is not a number";

    pnum="+"$num
    (echo "${pnum//[^\-+0-9]/}" | grep -Eq $rg) \
        && echo "$pnum is a number" \
        || echo "$pnum is not a number";

    npnum="-+"$num
    (echo "${npnum//[^\-+0-9]/}" | grep -Eq $rg) \
        && echo "$npnum is a number" \
        || echo "$npnum is not a number";
done

will output:

73 is a number
-73 is a number
+73 is a number
-+73 is not a number
foobar is not a number
-foobar is not a number
+foobar is not a number
-+foobar is not a number