Cannot replace keyword using PHP Regex

Question

I want to replace 'keyword' in <img ..keyword../> using PHP regular expressions.

$content = '<img alt="hello" src="http://frbird.qiniudn.com/topic/150609/5576a8837fd32e3b4ece5f6b-hd.jpg">';

$content = preg_replace('/(<img\s(?!>)*?)(hello)((?!>)*>)/U', '$1%&&&&&%$3', $content);

I can't replace 'hello'. Please tell me what is wrong.

Answer 1

Don't try to handle HTML (even escaped as it is) using a RegEx. Examples: Can you provide some examples of why it is hard to parse XML and HTML with a regex?

Parse the fragment using SimpleXML/DOM, and look for attributes that way.

<?php
$string = html_entity_decode('&lt;img alt="hello" src="http://frbird.qiniudn.com/topic/150609/5576a8837fd32e3b4ece5f6b-hd.jpg"&gt;');

$dom = new DOMDocument();

$dom->loadHTML(
    $string,
    LIBXML_COMPACT | LIBXML_HTML_NOIMPLIED | LIBXML_HTML_NODEFDTD
);

$attr = $dom->documentElement->removeAttribute('alt');

echo $dom->saveHTML();

Answer 2

First translate the < and > tokens from the source string into < and >. That way they are far easier to handle. Then do the preg_replace() and translate them back again in the target string.

$src= strtr('&lt;img alt="hello" src="http://frbird.qiniudn.com/topic/150609/5576a8837fd32e3b4ece5f6b-hd.jpg"&gt;',array('&lt;'=>'<','&gt;'=>'>'));
$trg = strtr(preg_replace('/(<img[^>]*")(hello)(".*)/', '$1xxxxx$3', $src),array('<'=>'&lt;','>'=>'&gt;'));
echo $trg;

Not very elegant but it will do the trick.