Shell Programming and Scripting

I need some help with the logic and syntax for a shell script (ksh) that will search a directory and look for similar files and save only the 5 versions. The version number is in the file name.
However, the files are of varying name lengths and may have 1 or many files, with no limit to the number of files.
the input file

Code:

A_14122012.txt
A_15122012.txt
A_16122012.txt
A_17122012.txt

the output should contain the present and past one file

Code:

A_16122012.txt
A_17122012.txt

i used the below code please sugest the correct script

Code:

for file in `ls *.txt | sort -r | nawk -F"[2012|.]" '{if(_[$1]<2){print $0;_[$1]++}}' `
do
    echo $file
done

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