PHP Regular Expression - String Replacement inside a Paragraph

Requirements: - Write a PHP function that accepts a needle (search keyword) and a haystack (paragraph of text). - Regular expression (preg_replace or similar) inside the function must do a case insensitive replacement of $keyword with <span class='keyword'>$keyword</span> --- Do not replace inside words --- Replace only when bordering characters of word are punctuation (.,;:!?), newline or space. Do not remove the punctuation. ---- Include test cases. Example before: The horse jumped over the fence. Example after: The horse jumped over the <span class="keyword">fence</span>. I wrote some REGEX tests which are not working properly, however these may give you some ideas: $text = preg_replace('/[^a-zA-Z0-9]' . $keyword . '[^a-zA-Z0-9]/i, " <span class='keyword'>".$keyword."</span>", $text) $text = preg_replace('/\b(?:[^\w,\n]+[a-z]+)*\b(' . $keyword . ')\b([^\w,\n])/i', " <span class='keyword'>".$keyword."</span>", $text) Project must be completed within 2 days. Please do not bid if you are unable to work within the timeframe. I will only work with developers directly. Any bids from salesmen will be declined.