Definition of function base_yylex in version 8.1.4

Hi all,

I am having a project in Postgres where I need to modify some code in the parser section of the code base. During some analysis I came across a function called "base_yylex" which is called in the function yylex in the file parser.c. However, I was unable to find the definition of this function due to which I am stuck completely.

Looking forward to some help from experts.

Thanks in advance