highlighting searchterms as bold text

["C.R.Vegelin" <cr.vegelin@xxxxxxxxx>]

Hi everyone,

I want to highlight (bold) searchterms in text.
For example, for all words starting with "ethyl":
a) replace "ethyl" by "<b>ethyl</b>"
b) replace "Ethyl" by "<b>Ethyl</b>"
c) replace "ethylene" by "<b>ethylene</b>"
d) but not "methyl" by "<b>methyl</b>"

Now I use:
$patterns[0] = "/ethyl/";
$replacements[0] = "<b>ethyl</b>";
$text = preg_replace($patterns, $replacements, $text);

This works for a) and c), but not for b) and d).
Any idea how to do this ?

TIA, Cor