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Asked: 2026-05-11T10:26:44+00:00

Given the following code : <body> <img src=source.jpg /> <p> <img src=source.jpg id =hello

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Given the following code :

<body>   <img src='source.jpg' />   <p>     <img src='source.jpg' id ='hello' alt='nothing' />     <img src='source.jpg' id ='world'/>   </p> </body>

What’s the best way – using a regular expression (or better?) – to replace it so it becomes this:

<body>   <img src='source.jpg' id='img_0' />   <p>     <img src='source.jpg' id ='img_1'  alt='nothing' />     <img src='source.jpg' id ='img_2'/>   </p> </body>

In other words :

  • All the <image /> tags all gets populated by an id attribute.

  • The id attribute should contain an incremented attribute (this is not really the problem though as its just part of the replace procedure)

I guess two passes are needed, one to remove all the existent id attributes and another to populate with new ones ?

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1 Answer

  1. I think the best approach is to use preg_replace_callback.

    Also I would recommend a slightly more stringent regexp than those suggested so far – what if your page contains an <img /> tag that does not contain an id attribute?

    $page = ' <body>   <img src='source.jpg' />   <p>     <img src='source.jpg' id ='hello' alt='nothing' />     <img src='source.jpg' id ='world'/>   </p> </body>';  function my_callback($matches) {     static $i = 0;     return $matches[1].'img_'.$i++; }  print preg_replace_callback('/(<img[^>]*id\s*=\s*')([^']*)/', 'my_callback', $page);

    Which produces the following for me:

    <body>   <img src='source.jpg' />   <p>     <img src='source.jpg' id ='img_0' alt='nothing' />     <img src='source.jpg' id ='img_1'/>   </p> </body>

    The regexp has two capturing groups, the first we preserve, the second we replace. I’ve used lots of negative character classes (e.g. [^>]* = up to closing >) to make sure that <img /> tags arn’t required to have id attributes.

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