Shell Programming and Scripting

        sub=$(expr substr "$line" 4 2)
        val=$(expr "$sub" : '\([0-9][0-9]\)')
        if [[ -n "$val" ]]; then
                echo " is a number"
        else
                echo " not a number"
        fi

#!/bin/ksh93

while read LINE
do
   sub=${LINE:4:2}
   case $sub in
      ( +([0-9]) )  echo "$sub is a number" ;;
      *)            echo "$sub is not a number" ;;
   esac
done < file

12 is a number
43 is a number
53 is a number
ST is not a number

num=${DDNAME:3:2}
if [[ $num =~ [0-9]+ ]]; then
  command $num
fi

sub=${line:3:2}

#!/bin/ksh
cat "num_str.txt" | while read line
do
        echo "line:$line"
        sub=${line:3:2}
        echo "sub:$sub"
        #sub=$(expr substr "$line" 4 2)
        #val=$(expr "$sub" : '\([0-9][0-9]\)')
        #if [[ -n "$val" ]]; then
        #       echo " is a number"
        #else
        #       echo " not a number"
        #fi
done

$ ksh num_str.ksh
line:E0801260
num_str.ksh[14]: : bad substitution