validate number and then format to decimal

I have a function in my script that dont work like I want. The goal is to validate numbers and then formating into decimal.

So I created a test-file thats looks like my function ...

#!/bin/bash
TMP_NUM="$1"
if [[ "$TMP_NUM" = *[[:digit:]]* ]]; then
    TMP_DECIMAL=$(awk -v decimal="$TMP_NUM" 'BEGIN {printf("%f", decimal) }')
    if [[ "$TMP_DECIMAL" =~ ^[0-9]+([.][0-9]+)?$ ]] ; then
        echo "$TMP_DECIMAL"
    else
        echo "Failed !"
    fi
else
    echo "failed !"
fi

exit 0

the results ...

suleiman@antec:~$ test.sh 0.123456789
0.123457

suleiman@antec:~$ test.sh 0.123456
0.123456

suleiman@antec:~$ test.sh Hello World
failed !

suleiman@antec:~$ test.sh 0Hello World
0.000000

suleiman@antec:~$ test.sh 0.00xyz01
0.000000

How do I have to change the test parameters to get only numbers formated, and an error message when any letters are behind any numbers, except eE ?

And why the hack my function kills some value in it, like in the first test result.

suleiman@antec:~$ test.sh 0.123456789
0.123457