L - Lex Source File is a file extension that can only be used with Lex. This program is used to generate lexical analyzers. It also consists of formal declarations that let users use different source codes in a certain file as well. You will also be able to convert text into a certain sequence and do much more. There is also a C source code that is generated by Lex that you will notice once you download this extension. It can also be accessed online. You will need to have all of this information available when it comes to compiling different source codes.