I am trying to pass a regular expression variable from a simple script to sed
to remove entries from a text file

e.g.

a='aaaa bbbb cccc ...|...:'

then executing sed from the script

sed s'/"'$a"'//g <$FILE > $FILE"_"1

my output file is always the same as the input file !!

any hints / pointers are aprreciated

eval should accomplish what you wish.

I am not to certain what you mean and how to pass eval the command value

Use awk instead of sed...

awk '{gsub(a,b);print}' a="$a" b="" file1 > file2

thanks for the hint, bit of modification got the script working

use
double qoutes instead of single quotes wil solve the problem
sed "s/$a//g" inputfile>tempfile;mv tempfile inputfile


will give the desired result