Edit in JSFiddle

function f2c(x)
{
  function convert(matched, p1, offset, str)
  {
    return ((p1-32) * 5/9) + "°C";
  }
  var s = String(x);
  var test = /(\d+(?:\.\d*)?)°F\b/g;
  return s.replace(test, convert);
}
document.write(f2c("32°F to 212°F"));