WordPress Posts Where Filter - Query by first character of post title

query-by-first-character.php

add_filter('posts_where', 'first_character_posts_where' );

function first_character_posts_where($where) {
		global $wpdb;
        
		//Take in a URL parameter
    $alpha_filter = sanitize_text_field( $_GET['alpha'] );
		
		// throw a different regex where at the database depending on if we want numbers or a specific character
		if( $alpha_filter !== false && $alpha_filter == 'number' ) {
			$where .= $wpdb->prepare( " $wpdb->posts.post_title REGEXP %s ", '^[0-9]' );
		} elseif( $this->alpha_filter !== false ) {
			$where .= $wpdb->prepare( " LOWER($wpdb->posts.post_title) REGEXP %s",'^'.$alpha_filter );
		}
		return $where;
	}