попробуйте это:
<code><?php
//the string to search in
$s = "<div style=\"font-size:15px;\">first matched here</div>";
//set up an empty array to facilitate the results
$matches = array();
//match with regex
//pattern has to start and end with the same character (of your choice)
//here it's #
//start criteria is an opening div <div> that may contain some text
//between v and >, so use .*?
//the dot means "any character", the *? means "any number, but as few as possible"
//then, define the part to capture. this is done with paranthesis
//define what to capture. this would be any character, and as few as possible
//and finally end criteria </div>
//matches are stored in the out parameter $matches
preg_match("#<div.*?>(.*?)</div>#", $s, $matches);
echo "<pre>";
print_r($matches);
echo "
";