Regular Expression Matching

Michael

Dec 10, 2016 • 1 min read

Implement regular expression matching with support for '.' and '*'.

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

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

The function prototype should be:
bool isMatch(const char *s, const char *p)

Some examples:
isMatch("aa","a") → false
isMatch("aa","aa") → true
isMatch("aaa","aa") → false
isMatch("aa", "a*") → true
isMatch("aa", ".*") → true
isMatch("ab", ".*") → true
isMatch("aab", "c*a*b") → true

public class Solution {
    public boolean isMatch(String s, String p) {
        if (p.length() == 0) {
            return s.length() == 0;
        }
        
        if (p.length() == 1 || p.charAt(1) != '*') {
            if (s.length() < 1) {
                return false;
            } else if (p.charAt(0) != s.charAt(0) && p.charAt(0) != '.') {
                return false;
            } else {
                return isMatch(s.substring(1), p.substring(1));
            }
        } else {
            if (isMatch(s, p.substring(2))) {
			    return true;
            }
		
		    int i = 0;
		    while (i < s.length() && (s.charAt(i) == p.charAt(0) || p.charAt(0) == '.')) {
			    if (isMatch(s.substring(i + 1), p.substring(2))) {
				    return true;
			    }
			    i++;
		    }
		    
		    return false;
        }
    }
}

Hope this helps,
Michael