如何使用正则表达式在标记a上添加属性rel nofollow

Question

how to add attribute rel nofollow on tag a with regex

Sample:

<a href="http://www.test.org/5521" rel="follow">test1</a>
<a href="http://www.test.org/5522" rel="external">test1</a>
<a href="http://www.test.org/5523">test1</a>

To:

<a href="http://www.test.org/5521" rel="nofollow">test1</a>
<a href="http://www.test.org/5522" rel="nofollow">test1</a>
<a href="http://www.test.org/5523" rel="nofollow">test1</a>

Answer 1

Step1: Remove All Rel from tag a

$result = preg_replace('@rel="(.*)"@U', '', $html);

Step2: Add Rel Nofollow on taf a

$result = preg_replace('@<a(.*)>@U', '<a$1 rel="nofollow">', $result);