正则表达式（PHP）查找以set字符串开头的所有并发

I am currently trying to make a game of ghost and this is my php file that uses regex to update a string in my html file that lists all letters that could possibly form a word following the string. (like for liche, it would echo n for possible letters) In addition, I have a text file with all words in the english dictionary that I am pulling data from for preg_match_all(). However, my code keeps returning nothing and I am not sure why.I am currently trying to make a game of ghost a