Regular Expression Matching

Regular Expression Matching

LeetCode Problem

• Problem Number: 10
• Problem Link: https://leetcode.com/problems/regular-expression-matching/
• Difficulty: Hard
• Topics: String, Dynamic Programming, Recursion

Problem Description

Given an input string s and a pattern p, implement regular expression matching with support for '.' and '*' where:

• '.' Matches any single character
• '*' Matches zero or more of the preceding element

The matching should cover the entire input string (not partial).

Note:

• For each appearance of '*', there will be a previous valid character to match
• The pattern must match the entire string, not just a portion of it

Examples

Example 1:

Input: s = "aa", p = "a"
Output: false
Explanation: "a" does not match the entire string "aa".

Example 2:

Input: s = "aa", p = "a*"
Output: true
Explanation: '*' means zero or more of the preceding element, 'a'. Therefore, by repeating 'a' once, it becomes "aa".

Example 3:

Input: s = "ab", p = ".*"
Output: true
Explanation: ".*" means "zero or more (*) of any character (.)".

Constraints

• 1 <= s.length <= 20
• 1 <= p.length <= 20
• s contains only lowercase English letters
• p contains only lowercase English letters, '.', and '*'
• It is guaranteed for each appearance of the character '*', there will be a previous valid character to match.