<p>Given an input string <code>s</code>&nbsp;and a pattern <code>p</code>, implement regular expression matching with support for <code>&#39;.&#39;</code> and <code>&#39;*&#39;</code> where:</p>

<ul>
	<li><code>&#39;.&#39;</code> Matches any single character.​​​​</li>
	<li><code>&#39;*&#39;</code> Matches zero or more of the preceding element.</li>
</ul>

<p>The matching should cover the <strong>entire</strong> input string (not partial).</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> s = &quot;aa&quot;, p = &quot;a&quot;
<strong>Output:</strong> false
<strong>Explanation:</strong> &quot;a&quot; does not match the entire string &quot;aa&quot;.
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> s = &quot;aa&quot;, p = &quot;a*&quot;
<strong>Output:</strong> true
<strong>Explanation:</strong> &#39;*&#39; means zero or more of the preceding element, &#39;a&#39;. Therefore, by repeating &#39;a&#39; once, it becomes &quot;aa&quot;.
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> s = &quot;ab&quot;, p = &quot;.*&quot;
<strong>Output:</strong> true
<strong>Explanation:</strong> &quot;.*&quot; means &quot;zero or more (*) of any character (.)&quot;.
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= s.length&nbsp;&lt;= 20</code></li>
	<li><code>1 &lt;= p.length&nbsp;&lt;= 20</code></li>
	<li><code>s</code> contains only lowercase English letters.</li>
	<li><code>p</code> contains only lowercase English letters, <code>&#39;.&#39;</code>, and&nbsp;<code>&#39;*&#39;</code>.</li>
	<li>It is guaranteed for each appearance of the character <code>&#39;*&#39;</code>, there will be a previous valid character to match.</li>
</ul>

class Solution {

    bool regex_recursive(string s, string p, int i, int j, string prev)
    {
        bool output;
        if(i==s.size())
        {
            if(j==p.size())
            return true;
        }
        if(j==p.size())
        {
            return false;
        }
        char toMatch = p[j];
        bool flag = false;
        if((j+1)<p.size())
        {
            if(p[j+1]=='*')
            flag = true;
        }
        if(!flag)
        {
            if(i==s.size()) return false;
            if(toMatch=='.')
            {
                output = regex_recursive(s,p,i+1,j+1,{toMatch});
                if(output) return output;
            }
            else if(s[i]==toMatch)
            {
                output = regex_recursive(s,p,i+1,j+1,{toMatch});
                if(output) return output;
            }
        }
        else
        {
            string pattern = {toMatch};
            pattern+="*";
            output = regex_recursive(s,p,i,j+2,pattern); //2 because character and *
            if(output) return output;
            if(prev==".*" or prev==pattern) return false; //This is to avoid cases where we have patterns like a*a*a* to go into unnecessary loops
            if(toMatch=='.')
            {
                while(i<s.size())
                {
                    output = regex_recursive(s,p,i+1,j+2,pattern);
                    if(output) return output;
                    i++;
                }
            }
            else
            {
                while(i<s.size() && s[i]==toMatch)
                {
                    output = regex_recursive(s,p,i+1,j+2,pattern);
                    if(output) return output;
                    i++;
                }
            }
        }
        return false;
    }
public:
    bool isMatch(string s, string p) {
        return regex_recursive(s,p,0,0,"");       
    }
};


19493