Using sed to replace text in file on Mac OSX

sed_replace.sh

sed_replace() {
  # require all 3 parameters
  if [ .$1 = . ] || [ .$2 = . ] || [ .$3 = . ]; then
    echo "Usage: sed_replace [oldString] [newString] [targetFile]"
    return 0
  fi
  local oldString=$1
  local newString=$2
  local targetFile=$3
  local temp=`mktemp -t sed_replace.XXXXXXXXX`
  chmod ug+rw $temp
  sed 's/'$oldString'/'$newString'/g' $targetFile > $temp
  mv $temp $targetFile
}

sed -i "${3##*.}" s/$1/$2/g $3