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function substitute {
    if [ -z "$1" -o -z "$2" ]; then
        echo "Usage: substitue FROM_STRING TO_STRING [OPTION]..."
        echo
        echo "Replace all occurances of FROM_STRING (a sed-compatible regular"
        echo "expression) with TO_STRING in all files for which ack-grep matches"
        echo "FROM_STRING."
        echo
        echo "Any additional options are passed directly to ack-grep (e.g.,"
        echo " --type=html would only run the substitution on html files)." 
        return 1
    fi
    # Escape forward slashes for sed
    FROM_STRING=${1/\//\\/}
    TO_STRING=${2/\//\\/}
    shift 2
    ack-grep -l --print0 "$@" "$FROM_STRING" | xargs -0 -n 1 sed -i -e "s/$FROM_STRING/$TO_STRING/g"
}