Write a Regex Engine Subset That Can Describe Token Patterns

Regex Patterns Describe Tokens

Lexical specifications are often written as regular expressions: [0-9]+ for integers, [a-zA-Z_][a-zA-Z0-9_]* for identifiers. Flex translates these to DFAs. Building a small regex matcher teaches you what Flex generates under the hood.

Subset: Concatenation, Alternation, Kleene Star

A minimal regex engine supports concatenation (ab), alternation (a|b), and Kleene star (a*). Compile the pattern to an NFA, subset-construct to a DFA, or interpret with a recursive matcher. Each match corresponds to a token pattern in your language.

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• Digit class [0-9] matches one decimal digit
• + means one or more of the preceding element
• * means zero or more
• Anchor patterns to full lexeme, not substring search

Connecting to real compilers

Production compilers embed this step inside a longer pipeline. GCC flows through cpp, cc1, assembly, and ld; Clang uses the driver, Sema, LLVM IR passes, and a target backend. LLVM bitcode, JVM bytecode, and WASM are other familiar IRs at the same layer. The exercise isolates one pass so you can test it alone before chaining it to the next stage.

Why for this exercise

You will implement a regex matcher subset that can describe token patterns for your lexer. This exercise asks you to parse simple regex syntax and test whether an input string matches, forming the foundation for generated or table-driven scanners.