How to find and replace the variable field="1.0.0" with field="$var" in a file?

Question

bash

How to find and replace the variable field="1.0.0" with field="$var" in a file?
do:
a=`cat ../t.properties`
sed -i.bak 's/field=""/filed="$a"/g' data.xml
But, something doesn't work as it should.

Answer 1

in 1.0.0 you have dots in addition to numbers.

sed -i.bak 's/field=\"[.0-9]*\"/field="$a"/g' data.xml