Ambiguous Grammar | Solved Example | Compiler Design

In this tutorial, we dive into Lexers, Parsers, and Ambiguous Grammars — key concepts at the heart of every compiler and interpreter.

We begin by understanding what a lexer is — how it breaks source code into tokens using regular expressions (REs) — and why parsers instead rely on context-free grammars (CFGs) to capture nested, hierarchical structures like conditionals and loops.

Then, we explore one of the most famous examples of ambiguity in programming languages: the “dangling else” problem. Using a simple grammar, we show how a single else can attach to two different if statements, creating two valid parse trees — and hence, ambiguity.

Finally, we resolve the problem step by step by rewriting the grammar into matched and unmatched forms, making it unambiguous and compiler-friendly. Along the way, we discuss how modern parsers and grammar tools like YACC or ANTLR handle such cases automatically.

By the end, you’ll clearly understand:
Why lexers use REs and parsers use CFGs
What makes a grammar ambiguous
How to fix ambiguity in language design

Perfect for anyone studying compiler design, formal languages, or syntax analysis.

📘 Looking for detailed notes, solved examples, and extended practice problems in computer science?
Join the Sigma Solver Learner Community here:
👉 https://quognitive.com/sigmasolver/co...

Chapters in the video:
0:00 Introduction
0:33 Problem Statement
1:20 How machines read code
2:31 What is Lexer
5:52 What is Parser
7:14 Ambiguity in Grammar
8:52 Identifying the Ambiguity
12:58 Removing the Ambiguity
15:05 Inference
15:50 Modifications
16:59 Conclusion

If you are facing any issues do let me know in the comment section below, I am here to help ❤️
If you found this video useful then please consider subscribing to my channel 🙏

Background Music Credits (in order of use)
Outro Music Credit:
Spirit by Sappheiros:
"Spirit by Sappheiros" is under a Creative Commons ( cc-by ) license
Music promoted by BreakingCopyright: https://bit.ly/sappheiros-spirit