From 8ed058a35e9747ad2cb131e7be9df679eb6e485f Mon Sep 17 00:00:00 2001
From: Atul-Kashyap <atulkashyap1208@gmail.com>
Date: Sun, 30 May 2021 22:27:02 +0530
Subject: [PATCH] Added Regular Expression Matching Program in CPP

---
 Code/C++/regular_expression_matching.cpp | 76 ++++++++++++++++++++++++
 string/readme.md                         |  1 +
 2 files changed, 77 insertions(+)
 create mode 100644 Code/C++/regular_expression_matching.cpp

diff --git a/Code/C++/regular_expression_matching.cpp b/Code/C++/regular_expression_matching.cpp
new file mode 100644
index 000000000..b2d678247
--- /dev/null
+++ b/Code/C++/regular_expression_matching.cpp
@@ -0,0 +1,76 @@
+//  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.
+
+#include <bits/stdc++.h>
+using namespace std;
+
+
+bool isMatch(string s, string p) {
+    // finding the length of string and pattern
+    int m = s.size(), n = p.size();
+    // Use dp vector is used to store the intermediate result
+    vector<vector<bool>> dp(m + 1, vector<bool>(n + 1, false));
+
+    dp[0][0] = true;
+    // When a star(*) is present, we may need to check many different suffixes of the text
+    // and see if they match the rest of the pattern.
+    for (int i = 0; i <= m; i++) {
+        for (int j = 1; j <= n; j++) {
+
+            if (p[j - 1] == '*') {
+                dp[i][j] = dp[i][j - 2] || (i && dp[i - 1][j] && (s[i - 1] == p[j - 2] || p[j - 2] == '.'));
+            }
+            else {
+                dp[i][j] = i && dp[i - 1][j - 1] && (s[i - 1] == p[j - 1] || p[j - 1] == '.');
+            }
+        }
+    }
+    return dp[m][n];
+}
+
+
+int main() {
+
+    // Input :
+    string s1 = "aa", p1 = "a";
+    cout << isMatch(s1, p1) << endl;
+
+    string s2 = "aa", p2 = "a*";
+    cout << isMatch(s2, p2) << endl;
+
+    return 0;
+}
+
+
+// Test Cases :
+
+// Input: s = "aa", p = "a"
+// Output: false
+// Explanation: "a" does not match the entire string "aa".
+// ---------------------------------------------------------------
+
+// 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".
+// ------------------------------------------------------------------
+
+// Input: s = "ab", p = ".*"
+// Output: true
+// Explanation: ".*" means "zero or more (*) of any character (.)".
+// ------------------------------------------------------------------
+
+// Input: s = "aab", p = "c*a*b"
+// Output: true
+// Explanation: c can be repeated 0 times, a can be repeated 1 time.
+// Therefore, it matches "aab".
+// -------------------------------------------------------------------
+
+
+
+// Time Complexity: O(TP), Let T and P be the lengths of the text and the pattern respectively.
+
+// Space Complexity: The only memory we use is the O(TP) boolean entries in our cache.
+//                   Hence, the space complexity is O(TP
diff --git a/string/readme.md b/string/readme.md
index 4705afc3b..a9b242d0f 100644
--- a/string/readme.md
+++ b/string/readme.md
@@ -28,6 +28,7 @@ A string is a class that contains a char array, but automatically manages it for
 * Manacher's algorithm (finding the longest palindrome in a string) ----> [Java](/Code/Java/longPalindrome.java)
 * Maximum occuring character in a string ---->[C++](/Code/C++/max_count.cpp)
 * Number of deletions to make a string pallindrome ---->[Python](/string/Number_of_deletions_to_make_pallindrome.py)
+* Regular Expression Matching ---->[C++](/Code/C++/regular_expression_matching.cpp)
 * Reverse a string ---->[C++](/Code/C++/reverse_string.cpp) | [Java](/Code/Java/reverse_string.java)
 * Revere a string using Stack ---->[C++](/Code/C++/reverse_a_string_using_stack.cpp)
 * Reverse Individual Words in String ----> [Java](/Code/Java/revindivstring.java)