Generate random string in PHP based on preg_match

Good day! I would like to create a program that would generate a random string based on my Regular expression. I'd love to hear any suggestion because i have no idea how to start. below is my regular expression using preg_match_all().

preg_match_all("/[a-z]*(L[a-z]+.)+[a-z]+(L[a-z]+.+[a-z]+)*/"

Thanks!.