Add regular expression matching algorithm.

src/algorithms/string/regular-expression-matching/README.md

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+# Regular Expression Matching
+
+Given an input string `s` and a pattern `p`, 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).
+
+**Note**
+
+- `s` could be empty and contains only lowercase letters `a-z`.
+- `p` could be empty and contains only lowercase letters `a-z`, and characters like `.` or `*`.
+
+## Examples
+
+**Example #1**
+
+Input:
+```
+s = 'aa'
+p = 'a'
+```
+
+Output: `false`
+
+Explanation: `a` does not match the entire string `aa`.
+
+**Example #2**
+
+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`.
+
+**Example #3**
+
+Input:
+
+```
+s = 'ab'
+p = '.*'
+```
+
+Output: `true`
+
+Explanation: `.*` means "zero or more (`*`) of any character (`.`)".
+
+**Example #4**
+
+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`.
+
+## References
+
+- [YouTube](https://www.youtube.com/watch?v=l3hda49XcDE&list=PLLXdhg_r2hKA7DPDsunoDZ-Z769jWn4R8&index=71&t=0s)
+- [LeetCode](https://leetcode.com/problems/regular-expression-matching/description/)