入力内容が英数字だけかどうかチェックする

#!/bin/sh
#validalphanum.sh

validAlphaNum(){
    #入力内容をsedで加工しその結果を元の入力内容と比較する
    compressed="$(echo $1 | sed -e 's/[^[:alnum:]]//g')"
    if [ "$compressed" != "$1" ]; then
        return 1
    else
        return 0
    fi
}

echo -n "Enter input: "
read input
if ! validAlphaNum "$input" ; then
    echo "Your input must consist of only letters and numbers." >&2
    exit 1
else
    echo "Input is valid."
fi
exit 0

$ ./validalphanum.sh
Enter input: 1
Input is valid.

$ ./validalphanum.sh
Enter input: a
Input is valid.

$ ./validalphanum.sh
Enter input: ;
Your input must consist of only letters and numbers.