How do I use sed to change my configuration files, with flexible keys and values?

sed -i -e '/central\.database =/ s/= .*/= new_value/' /path/to/file

Explanation:

• -i tells sed to save the results to the input file. Without it sed will print the results to stdout.
• /central\.database =/ matches lines that contain the string between slashes: central.database =. The . is escaped since it's a special character in regex.
• The s/OLD/NEW/ part performs a substitution. The OLD string is a regular expression to match and the NEW part is the string to substitute in.
• In regular expressions, .* means "match anything". So = .* matches an equal sign, space, and then anything else afterward.

Here's an example expression:

sed -i 's/^\(central\.database\s*=\s*\).*$/\1SQLTEST/' file.cfg

If you want to match stuff with / in it, you can use another delimiter:

sed -i 's#^\(cent/ral\.data/base\s*=\s*\).*$#\1SQL/TEST#' file.cfg

Or with variable expansion:

VAL="SQLTEST"
sed -i "s/^\(central\.database\s*=\s*\).*\$/\1$VAL/" file.cfg

In your example:

sshRetValue=`sed -i "s/^\(\1$CENTRAL_DB_NAME\s*=\s*\).*\$/\1$CENTRAL_DB_VALUE/" /home/testing.txt`;

There's a \1 before $CENTRAL_DB_NAME that's invalid. Also, sed doesn't print it's return value. This is the preferred way to check return values:

sed -i "s/^\($CENTRAL_DB_NAME\s*=\s*\).*\$/\1$CENTRAL_DB_VALUE/" /home/testing.txt;
sed_return_value=$?

And ultimately piping to ssh (not tested):

sed_return_value=$(ssh server <<EOF
    sed -i "s/^\($CENTRAL_DB_NAME\s*=\s*\).*\$/\1$CENTRAL_DB_VALUE/" /home/testing.txt;
    echo $?
EOF
)

The -i is for replacing data in the input file. Otherwise sed writes to stdout.

Regular expressions are a field of their own. It would be impossible to explain them in depth in a stackoverflow answer, unless there is some specific function that's eluding you.