Web Fetch & Process

Hi How can I parse HTML page which I fetched by PHP so some part of text would be replaced by other text. For example, if I have bold text on HTML page (<b>Some text</b>) how can I manage that all bold text on that page is replaced by some other text. Also, I managed to delete all HTML code from page (with this code $data[0] = preg_replace("/([<])+([^>])+([.])*([>])+/i","", $data[0]); but also would like to delete all chars except numbers from page, can this be done? Regards. Zoran