regular-expression-matching.js

/**
10. Regular Expression Matching

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 <= 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.
*/

/**
 * @param {string} s
 * @param {string} p
 * @return {boolean}
 */
var isMatch = function (s, p) {
  if (p.length === 0) {
    return s.length === 0;
  }

  const isFirstMatched = s.length > 0 && (s[0] === p[0] || p[0] === ".");

  if (p.length >= 2 && p[1] === "*") {
    return (
      isMatch(s, p.substring(2)) ||
      (isFirstMatched && isMatch(s.substring(1), p))
    );
  } else {
    return isFirstMatched && isMatch(s.substring(1), p.substring(1));
  }
};