exit with error message in bash (oneline)

exit doesn't take more than one argument. To print any message like you want, you can use echo and then exit.

    [[ $TRESHOLD =~ ^[0-9]+$ ]] || \
     { echo "Threshold must be an integer value!"; exit $ERRCODE; }

You can use a helper function:

function fail {
    printf '%s\n' "$1" >&2 ## Send message to stderr.
    exit "${2-1}" ## Return a code specified by $2, or 1 by default.
}

[[ $TRESHOLD =~ ^[0-9]+$ ]] || fail "Threshold must be an integer value!"

Function name can be different.

Using exit directly may be tricky as the script may be sourced from other places. I prefer instead using subshell with set -e (plus errors should go into cerr, not cout) :

set -e
[[ $TRESHOLD =~ ^[0-9]+$ ]] || \
     (>&2 echo "Threshold must be an integer value!"; exit $ERRCODE)