The Y extension is given to files associated with a parser generator program called Yacc (Yet Another Compiler-Compiler). Y files are used to create parsers that support various programming languages, but in the case of Yacc it is C language. Y is a text file that consists of declarations, grammar rules and source code written in the C programming language, but the with rules and the other two alternatively.

A parser is a parser that allows you to transform, for example, program code into code understandable for a compiler.