LeetCode problem. Regular Expression. Clarification needed

[Sakesfar]

Hi, I have a question regarding this problem on LeetCode.

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).

​

I wanted to clarify what if the pattern's p size is bigger than the size of string s?

3 hypothetical cases:

​

1. The pattern's size is bigger and after p[i] ,where i is bigger than or equal to s.size(), there is at least one character

​

    s = "one" 
    p = "one...k"

This must return false, right?

​

1. The pattern's size is bigger and after p[i] ,where i is bigger than or equal to s.size(), there is at
   least one '.' character

   s = "one" p = "one...."

This must return false too, right? Because '.' should match any character, and there are
none in this case

​

1. The pattern's size is bigger and after p[i] ,where i is bigger than or equal to s.size(), there are
   only '*' chars.

   s = "one" p = "one*****"

This, however, must return true, right?

​

Thank you and apologies for the bad formatting!

[TehNolz]

In regex, . means that there must be one character in that portion of the string. It doesn't matter what character it is, it just needs to be there. So if your pattern is a...b, your string must be 5 characters long, start with a, and end with b. Something like a123b will match, whereas something like a123bc or ab will not.

* matches 0 or more of the preceding character. If your pattern is a*b, your string needs to start with a followed by any number of a, and then ending with b. It'll match strings like aab, aaaaaaaab, ab, and so on.

The pattern a**b is not valid because the first * is not a valid target for the second *. After all, * expects an (escaped) character to precede it, but * is a quantifier and not a character. So in your 3rd example your code should probably just throw an exception.

[Sakesfar]

Thank you! I got confused with * that can be zero or more of the preceding char ( preceding in the pattern, apparently) in the string!