Regular Expression Matching

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

'.' Matches any single character.

'*' Matches zero or more of the preceding element. "a/star" means zero to any number of a

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", "cab") → true

https://leetcode.com/problems/regular-expression-matching/

Solution I:

public boolean isMatch(String s, String p) { // base case if (p.length() == 0) { return s.length() == 0; } // special case if (p.length() == 1) { // if the length of s is 0, return false if (s.length() < 1) { return false; } //if the first does not match, return false else if ((p.charAt(0) != s.charAt(0)) && (p.charAt(0) != '.')) { return false; } // otherwise, compare the rest of the string of s and p. else { return isMatch(s.substring(1), p.substring(1)); } } // case 1: when the second char of p is not '' if (p.charAt(1) != '') { if (s.length() < 1) { return false; } if ((p.charAt(0) != s.charAt(0)) && (p.charAt(0) != '.')) { return false; } else { return isMatch(s.substring(1), p.substring(1)); } } // case 2: when the second char of p is '', complex case. else { //case 2.1: a char & '' can stand for 0 element if (isMatch(s, p.substring(2))) { return true; } //case 2.2: a char & '*' can stand for 1 or more preceding element, //so try every sub string 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; } }

Solution II: concise version

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