Looping input command - shell

Question

How can we have input from user in DD/MM/YYYY format in shell code such that till user does not provides input in this format the same prompt is printed again and again.
printf "Please Enter START Date(DD/MM/YYYY)\n"
read date1
how to check in this that if date is provided in DD/MM/YYYY format or not. and if not then how to start this loop again?

Answer 1

This solution firstly checks the syntax of all the tokens
of the input and then using the command date checks if the
the input is a valid date. For instance 30/02/2014 has
a correct syntax but it is not a valid date. Please note
that the date command accepts the format MM/DD/YYYY.
#!/bin/bash
while [ 1 ]; do
IFS="/" read -p "Please Enter START Date(DD/MM/YYYY): " d m y
if [[ $d != [0-9][0-9] ]]; then echo "Day format invalid"; continue; fi
if [[ $m != [0-9][0-9] ]]; then echo "Month format invalid"; continue; fi
if [[ $y != [0-9][0-9][0-9][0-9] ]]; then echo "Year format invalid"; continue; fi
if date -d "$m/$d/$y" > /dev/null; then
date1="$d/$m/$y"
break
fi
done
# Here you can use $date1 as you prefer. For instance:
echo $date1

Answer 2

Try this one:
until read -p "Please Enter START Date (DD/MM/YYYY): " && [[ $REPLY == [0-9][0-9]/[0-9][0-9]/[0-9][0-9] ]]; do
## echo "Please enter in proper (DD/MM/YYYY) format."
continue
done
The echo part is just optional that you could consider adding.
By default, read places input in $REPLY variable if no variable is specified, but you could have a custom one if you like.
read -p "Please Enter START Date (DD/MM/YYYY): " INPUT && [[ $INPUT == ...
For a bit more sh-compatible and based on your own format you can have this:
#!/bin/sh
while :; do
printf "Please Enter START Date(DD/MM/YYYY)\n"
read date1
case "$date1" in
[0-9][0-9]/[0-9][0-9]/[0-9][0-9])
break
;;
esac
done