# Regular Expression Matching Leetcode Solution

## Regular Expression Matching Leetcode Problem

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).

## Regular Expression Matching Leetcode Solution :

### 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.

### Example 1:

Input: s = “aa”, p = “a*”

Output: true

### Example 2:

Input: s = “ab”, p = “.*”

Output: true

In the “Two Sum” problem, you are 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.

Approach :

For Solving two sum Leetcode Problem we can use following procedure :

• If the pattern is empty, return True if the text is also empty (base case).
• Check if the first characters of the text and pattern match, considering ‘.’ as a wildcard character.
• If the next character in the pattern is ”, there are two possibilities:
a. Ignore the ” and the preceding character in the pattern and continue matching the text with the shortened pattern.
b. Keep the preceding character in the pattern and match the next character in the text.
• If there is no ‘*’ in the pattern, simply match the next characters in the text and pattern.

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