Practice: Regular Expression Matching

Problem: https://leetcode.com/problems/regular-expression-matching/

Recognition reminder: you match a string s against a pattern p where . matches any single character and * matches zero or more of the element just before it. The subproblem is “does the first i of s match the first j of p,” two positions, one in each, so this is 2-D string DP. The hard part is the *, which forces a two-branch transition (zero copies, or one more copy); the week README’s core concepts name both branches. This is the hardest problem of the week; expect to lean on the framework.

Before you start (the five-beat rhythm)

  1. Your Pattern Card for this week is already written.
  2. Name the pattern aloud and write your approach as a plain-English comment before any code.
  3. Struggle floor: 25 minutes unaided. No hints, no AI, no Discuss tab.
  4. If stuck past the floor, ask the tutor for a hint. Six rungs, one per ask.
  5. Debrief in your commit message before moving to the next problem.

Your target

Fill these in yourself before you look at anyone else’s solution:

Target time complexity:  ____
Target space complexity: ____

The * case is a disjunction: be able to state, in plain English, what “match zero of the preceding element” reads and what “match one more” reads, and when the second branch is even allowed.

Where your code goes

Write your solution in your own work repo (see getting-started.md), not in this folder. This folder ships only the problem spec and a provided-example test (tests/test_provided.py) so you can check the given cases locally before you submit to LeetCode’s judge. The judge is the oracle; the tutor will not confirm your answer by reading it.

Debrief (paste into your commit message) 

1. What pattern did this turn out to be?
2. What was the trigger phrase or input shape that should have made me reach for it?
3. What was the time and space complexity, and what would dominate at scale?
4. What edge case would have broken my first attempt?
5. What would I do differently in three days when I see this cold?