Regular Expression Matching
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🕉Problem Statement:
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.
The matching should cover the entire input string (not partial).
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".
Example 5:
Input: s = "mississippi", p = "mis*is*p*."
Output: false
Solution:
Since this problem involves two strings ( (1) given string, (2) pattern or regular expression ), we already know from 2strings DP Problems chapters that there is a possibility that we would be able to use Suffix Approach where for every pair of possible substrings (from two given strings) we compare the last characters and take action appropriately.
Lets's assume the given string is str[0....n] and given regular expression is regexp[0...m], and let's assume dp[i][j] gives result for str[0...i] and regexp[0...j] .

What happens if character at regexp[m] is '.' ?
'.' matches one single character. It does not care about what the character is. str[0...n] will match regexp[0...m] as long as str[0...(n  1)] has already matched regexp[0...(m  1)]. For example: "hackathon" will match regular expression say "hackatho." as long long as "hackatho" has matched with regular expression "hackatho".
This leads us to the recurrence relation:
dp[n][m] = dp[n  1][m  1]

What happens when character at regexp[m] is '*' ?
'*' matches zero or more of the preceding character.
Now what this preceding character might be ? Anything including '.'
'.*' is different from all other characters as preceding character since '.' represents any character.
What I mean by this is: "a*" will be match "", "a", "aa", "aaa" and so on. Observe that how 'a' is fixed here, no other characters allowed since preceding character is 'a'.
This is not the case if preceding character is '.'. Since '.' represents any character, ".*" will match "", "a", "z", "hedhrekjfhkerjhfrekjfhrekjhfrkj" etc. Observe that we are not bound to repeat the same character since '.' represents any character.
So let's handle this case by dividing it into two scenarios:

Zero Occurrence of preceding character:
If character at index m in regular expression is '*', and the preceding character has not occurred even once in the given string then whether the string str[0...n] matches regexp[0...m] or not depends on whether str[0...n] has matched regexp[0...(m  2)] since regexp[m  1] = preceding character and regexp[m] = '*'.
This leads us to the recurrence relation:
dp[n][m] = dp[n][m  2] && regexp[m] == '*'

One or More Occurrences of preceding character:
str[0...n] will match regexp[0...m] as long as str[0...(n  1)] has matched regexp[0...m].
You might ask: okay, this is all good for multiple occurrences of the preceding character, but what about for only one occurrence of preceding character ? Should str[0...(n  1)] match with regexp[0...(m  2)] since we have only one occurrence ?
The answer is: yes you are correct in saying that, but "str[0...n] will match regexp[0...m] as long as str[0...(n  1)] has matched regexp[0...m]" already captures that since for one occurrence when we again compare str[0...(n  1)] with regexp[0...m] (instead of regexp[0...(m2)]) we would get correct result since it would become a case for zero occurrence.
This leads us to the recurrence relation:
dp[n][m] = dp[n  1][m] && regexp[m  1] == str[n]
Here the special case is: when character at regexp[m  1] is '.'
We would still get a very similar recurrence relation as above, but since '.' represents any character, instead of checking for whetherregexp[m  1] == str[n]
, we would be checking for whetherregexp[m  1] == '.'
This leads us to the following recurrence relation:
dp[n][m] = dp[n  1][m] && regexp[m  1] == '.'

What happens when character at regexp[m] is neither '.' nor '*' ?
Say regexp[m] is 'a' or 'y' or '7' or any character but not '.' or '*'.
In this case we need to check: whether
str[n] == regexp[m]
 whether str[0...(n  1)] matches regexp[0..(m  1)]
So, we get the following recurrence relation:
dp[n][m] = dp[n  1][m  1] && str[n] == regexp[m]
 whether

Zero Occurrence of preceding character:
The above algorithm can be very easily implemented using Bottomup Dynamic Programming Approach. We would do tabulation and the dimension of the table would be
(length of given string + 1) x (length of regular expression + 1)
. We are adding one in the
dimension because the dimensions represent
length of the substrings and not index. dp[i][j] would represent result
for given string str[0...(i  1)] and regular expression regexp[0...(j  1)].
i = 0 would represent empty string and j = 0 would represent empty regular expression.

dp[0][0] is always true since empty string matches empty regular expression.

If regular expression is empty and given string is nonempty,
we would always get false, since they don't match.
So, dp[i][0] is always false if i != 0.

If regular expression is nonempty and string is empty,
then they would match only if regular expression is
of the form "a*" or "a*b*" and so on, since '*' represent zero or more occurrences.
If regular expression contains '.', the the regular expression cannot match an empty string, since '.' represents one occurrence of any character.
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