# 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;
}
}
}
``````