Does anyone know how to fix that?

A quick way to do that with a regular expression would be :

$('p.someclass').each(function() {
  var original = $(this).html();
  $(this).html( original.replace(/-/g, '—') );
});

Keep in mind, though, that this will mess up IDs or class names that have hyphens in them, if they’re in those paragraphs. I don’t have time to work out how to avoid that at the moment, but if someone else would like to give it a shot, go for it.

What if I wanted to change all hyphens to em dashes? And better yet in a specified paragraph class?

Is this possible? How would you do that?

My money’s on the second (string-based) method as the best performing approach, but it would be helpful to provide benchmark results in your post so that each solution’s performance can be taken into account…

And I wonder what the best way would be of ensuring that if any of the three approaches are accidentally (or intentionally) run multiple times the changes are not reapplied each time, resulting in new lines of 3,4,5,etc words…?

  $('h2').each(function() {
    var h2Contents = $(this).html().split(" ");
    h2Contents[h2Contents.length-2] = '<span style="white-space: nowrap; color: red;">' + h2Contents[h2Contents.length-2];
    h2Contents[h2Contents.length-1] = h2Contents[h2Contents.length-1] + '</span>';
    $(this).html(h2Contents.join(' '));
  });

Good catch there. Yeah, I didn’t account for nested tags in the headings. If you were to do that, you’d need to change h2all = $(this).text(); to h2all = $(this).html();.

<h4>heading goes here <em> July 7th 2008</em> </h4> 

with some style applied to it then the inline element loses its style. I don’t know exactly what happens but it bypasses the styles applied to that inline element. I thought I’d point it out since I just ran into this.