A Lex Yacc developer designs and implements compilers and interpreters using Lex and Yacc tools. Lex is used for lexical analysis; it breaks down input into meaningful sequences called tokens. Yacc, yet another compiler compiler, is used for syntax analysis; it analyzes these tokens in context, checking for correct syntax and building a parse tree. The developer uses these tools for tasks like programming language development, data validation, configuring systems, or creating domain-specific languages. They write grammar rules, handle errors, and optimize for performance. Their work is critical in translating high-level code into machine-understandable instructions.