$string = "<img src=\"test.jpg\"> highlight the word 'word' in this sentence";

##</code><code>##

<img src="test.jpg"> highlight the <b>WORD</b> '<b>WORD</b>' in this sentence

##</code><code>##

$string "<img src=\"test.jpg\"> highlight the word <img src=\"word.jpg\">";

##</code><code>##

<img src="test.jpg"> highlight the <b>WORD</b> <img src="word.jpg">;

##</code><code>##

$replacementString = "<b>WORD</b>";
$string =~ s/word/$replacementString/ig;