$re = '/alt.+?(?= src)/m';
$str = '<img alt="" border="0" width="320" data-original-height="2448" data-original-width="3264" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEioWN9OlO4KRxREJkSozaN1rB0DlDTEnMnO5yh3zzeR-ipfXsoQC2KIjOA6_HzNBqqzhS6UxAaobig49GlOFaI7-P5jHmibtpmBJW6iuLrHcYtdC6Em6RWNzIOlfshFkpDgv_jE5VcHet5xxxYE9eZJ157b-bytHBq4pA78kPdoUeXn9o55eHa4OjPE_g/s320/1670258048910-01.jpeg"/>
<img alt="" border="0" width="320" data-original-height="3472" data-original-width="4624" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgmJSEhOjEIyY-T0IYWeyPqFxWL14L6rhbR733dI9wm0tvVSKXtRU0iC2FkNxKgprqUUtiLG6PFyGZ-pSWsPNuH99VQsn-O3zPeZvIGRUIY7Y-lIYiFeAhLAGhXVQZE-FGWQ8ep4wpzbCanwI-SlphDIKkB9gzgEXfLyw1uQ9iwDDAGOXLINasbRCxADA/s320/1670267239853-01.jpeg"/>';
$subst = "border=\"0\"";
$result = preg_replace($re, $subst, $str);
echo "The result of the substitution is ".$result;
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php