remove_wrapping_p_tag.php

<?php
/**
 * Move image inside <p> tag above the <p> tag while preserving any link around image.
 * Can be prevented by adding any attribute or whitespace to <p> tag, e.g. <p class="yolo"> or even <p >
 */
function gc_remove_p_tags_around_images($content)
{
	$contentWithFixedPTags =  preg_replace_callback('/<p>((?:.(?!p>))*?)(<a[^>]*>)?\s*(<img[^>]+>)(<\/a>)?(.*?)<\/p>/is', function($matches) 
	{
		/*
		Groups 	Regex 			Description
			<p>			starting <p> tag
		1	((?:.(?!p>))*?)		match 0 or more of anything not followed by p>
			    .(?!p>) 		anything that's not followed by p>
			  ?: 			non-capturing group. 
				    *?		match the ". modified by p> condition" expression non-greedily
		2	(<a[^>]*>)?		starting <a> tag (optional)
			\s*			white space (optional)
		3	(<img[^>]+>)		<img> tag
			\s*			white space (optional)
		4	(<\/a>)? 		ending </a> tag (optional)
		5	(.*?)<\/p>		everything up to the final </p>

			i modifier 		case insensitive
			s modifier		allows . to match multiple lines (important for 1st and 5th group)
		*/
			
		// image and (optional) link: <a ...><img ...></a>
		$image = $matches[2] . $matches[3] . $matches[4];

		// content before and after image. wrap in <p> unless it's empty
		$content = trim( $matches[1] . $matches[5] );
		if ($content) {
			$content = '<p>'. $content .'</p>';
		}

		return $image . $content;

	}, $content);
	
	// On large strings, this regular expression fails to execute, returning NULL
	return is_null($contentWithFixedPTags) ? $content : $contentWithFixedPTags;
}
add_filter('the_content', 'gc_remove_p_tags_around_images');