Regex can be an effective first stage for lexers when you control token priority and input boundaries. The key is designing rules that are deterministic enough for linear scanning.

Longest Match + Rule Priority

Most lexer implementations choose either “max munch” (longest token wins) or fixed rule priority (first match wins). Document the strategy and keep it consistent.

Separate Trivia Tokens

Model whitespace and comments explicitly. Keeping them as removable “trivia” tokens simplifies downstream parsing and source mapping.

Avoid Ambiguous Catch-All Rules

Broad rules like .+ can hide tokenizer bugs. Prefer explicit character classes and clear fallback error tokens.

Benchmark on Real Files

Tokenizers may pass micro-tests but degrade on large projects. Measure token throughput and worst-case files before production rollout.