This is part 1 of a series that will show you how to write an interactive interpreter in Object Pascal. Part 1 of the series will cover introductory material including a description of the language we'll create, a full lexical analyzer for the language, how to use DUnitX for unit testing, and an introduction to the essential concepts in syntax analysis, recursive descent, grammar, and EBNF. Along the way, we'll create a REPL, give a detailed discussion of how to parse expressions and build a simple interactive calculator to illustrate the theory. The book provides fully working code and explains in plain English how the code works and why certain decisions were made, including alternative designs. The book makes liberal use of code throughout the book chapters. Everything is done without the help of third-party tools. All you need is a standard installation of Free Pascal or Embarcaderos's excellent Delphi (including the free community edition). The text is geared to hobbyists and midlevel developers who need an easy introduction to lexical analysis and parsing. It's also for students starting out in compiler and interpreter design and need something more digestible before getting immersed in compiler theory.