Source

File: wp-includes/class-wp-embed.php

public function autoembed( $content ) {
		// Replace line breaks from all HTML elements with placeholders.
		$content = wp_replace_in_html_tags( $content, array( "\n" => '<!-- wp-line-break -->' ) );

		if ( preg_match( '#(^|\s|>)https?://#i', $content ) ) {
			// Find URLs on their own line.
			$content = preg_replace_callback( '|^(\s*)(https?://[^\s<>"]+)(\s*)$|im', array( $this, 'autoembed_callback' ), $content );
			// Find URLs in their own paragraph.
			$content = preg_replace_callback( '|(<p(?: [^>]*)?>\s*)(https?://[^\s<>"]+)(\s*<\/p>)|i', array( $this, 'autoembed_callback' ), $content );
		}

		// Put the line breaks back.
		return str_replace( '<!-- wp-line-break -->', "\n", $content );
	}