# 10.regular-expression-matching

## Statement

• Difficulty: Hard
• Tag: `递归` `字符串` `动态规划`

• `'.'` 匹配任意单个字符
• `'*'` 匹配零个或多个前面的那一个元素

``````输入：s = "aa", p = "a"

``````

``````输入：s = "aa", p = "a*"

``````

``````输入：s = "ab", p = ".*"

``````

• `1 <= s.length <= 20`
• `1 <= p.length <= 30`
• `s` 只包含从 `a-z` 的小写字母。
• `p` 只包含从 `a-z` 的小写字母，以及字符 `.` 和 `*`
• 保证每次出现字符 `*` 时，前面都匹配到有效的字符

• Link: Regular Expression Matching
• Difficulty: Hard
• Tag: `Recursion` `String` `Dynamic Programming`

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).

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 <= 30`
• `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.