Add Bootstrap image classes to WordPress media attachments

add_bootstrap_class_to_images.php

/**
 * Add the following to your theme's `functions.php` to automatically add `img-fluid` class
 * to all images inserted using the "Add Media" upload button in WordPress editor.
 */
if ( ! function_exists( 'add_bootstrap_class_to_images' ) ) {
	function add_bootstrap_class_to_images( $html, $attachment_id, $attachment ) {
		$img_element = "/<img[^>]*>/";
		$found = preg_match( $img_element, $html, $image );

		// If no image, do nothing
		if ( $found <= 0)
			return $html;

		$image = $image[0];
		
		if ( strstr( $image, "class=\"" ) !== FALSE ) { // If image already has class defined inject it to attribute
			$image_new = str_replace("class=\"", "class=\"img-fluid ", $image);
			$html = str_replace($image, $image_new, $html);
		} else { // If no class defined, just add class attribute
			$html = str_replace("<img ", "<img class=\"img-fluid\"", $html);
		}
		return $html;
	}
	add_filter( 'image_send_to_editor', 'add_bootstrap_class_to_images', 10, 3 );
}