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Regex and rules still matter

Deterministic **rules and regular expressions** catch crisp structure—IDs, formats, allowlists, required legal phrases—either instead of ML or as **gates** around ML/LLM systems. SLP3’s early chapters on words/tokens and classic classifi...

What it is

Deterministic rules and regular expressions catch crisp structure—IDs, formats, allowlists, required legal phrases—either instead of ML or as gates around ML/LLM systems. SLP3’s early chapters on words/tokens and classic classifiers sit beside modern LLMs for a reason: not every problem is fuzzy language.

<!-- IMAGE: rule gate before/after model -->

MEDIUM PRIORITYFLOWCHART
◷ IN PRODUCTION

Visual Spec & Architecture Diagram

Decision tree: structured pattern (dates, IDs) → regex/rules; fuzzy semantics → ML. Hybrid box in middle. Example fake invoice line with regex capture groups highlighted.

Educational Focus: Teaches when not to use a model.

Why it matters

Hybrid systems are adult engineering. Rules enforce invariants models approximate poorly (invoice shapes, jurisdiction codes, “must include risk disclosure”). They are testable like code.

How it works (plain)

  1. List invariants that must never be “model-guessed.”
  2. Implement rules/regex with unit tests.
  3. Use ML/LLM for fuzzy language around those rails.
  4. Log rule hits vs model decisions for audits.
  5. Retire rules that became unmaintainable—don’t let the pile rot.

CS224N focuses on neural methods; production NLP still needs this chapter’s discipline when shipping.

Everyday example

Reject any “account number” that fails a known checksum before an LLM drafts a support reply.

Try it

List three checks in your domain that should never be model-guessed. Write one regex or deterministic function for the easiest one, plus two failing tests.

Myths

⚠️ Myth: Deep learning made rules obsolete.
✓ Reality: Rules enforce invariants models approximate poorly.
⚠️ Myth: More regex always means more safety.
✓ Reality: Unmaintainable piles create blind spots.
⚠️ Myth: LLMs can replace validators.
✓ Reality: Validators should be boring and exact.

Sources