Source

File: wp-includes/formatting.php

function tag_escape( $tag_name ) {
	$safe_tag = strtolower( preg_replace( '/[^a-zA-Z0-9_:]/', '', $tag_name ) );
	/**
	 * Filters a string cleaned and escaped for output as an HTML tag.
	 *
	 * @since 2.8.0
	 *
	 * @param string $safe_tag The tag name after it has been escaped.
	 * @param string $tag_name The text before it was escaped.
	 */
	return apply_filters( 'tag_escape', $safe_tag, $tag_name );
}