Regular expression matching

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).

Input and output 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 <= 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.

Solution

class Solution(object):
    def isMatch(self, s, p):
        """
        :type s: str
        :type p: str
        :rtype: bool
        """        
        if not p:
            return not s
        first_match = bool(s) and p[0] in {s[0], '.'}
        if len(p) >= 2 and p[1] == '*':
            return self.isMatch(s, p[2:]) or first_match and self.isMatch(s[1:], p)
        else:
            return first_match and self.isMatch(s[1:], p[1:])

Test the the solution:

s = "aa"
p = "a"
print(Solution().isMatch(s, p))

s = "aa"
p = "a*"
print(Solution().isMatch(s, p))

s = "ab"
p = ".*"
print(Solution().isMatch(s, p))

s = "aab"
p = "c*a*b"
print(Solution().isMatch(s, p))

s = "mississippi"
p = "mis*is*p*."
print(Solution().isMatch(s, p))

More interview questions can be found here:
Algorithm