/m treats the text as multiple lines - meaning that each line is viewed separately. Obviously you don't have any line that has both the opening and closing blockquote tag.
/s treats the text as one single line. Now the opening and closing tags meet each other.
Snippets of code should be wrapped in
<code> tags not<pre> tags. In fact, <pre>
tags should generally be avoided. If they must
be used, extreme care should be
taken to ensure that their contents do not
have long lines (<70 chars), in order to prevent
horizontal scrolling (and possible janitor
intervention).