正则表达式，负向前瞻（？！？）发现<标签后面没有？

How to find < tags not followed by ?

$htmlStr = " ba <div>b <? </div>n";
$regex1 = '#<#'; // finds 3 '<' 
$regex2 = '#<(?!?)#'; // does not find anyhting, although should find two '<' not followed by '?' 

? is a special character in your regexp pattern and should be escaped:

$htmlStr = " ba <div>b <? </div>n";
$regex2 = '#<(?!\?)#';   // <-- will find 2 matches