Check if a binary string has a 0 between 1s or not | Set 2 (Regular Expression Approach)

Given a string of 0 and 1, we need to check that the given string is valid or not. The given string is valid when there is no zero is present in between 1's. For example, 1111, 0000111110, 1111000 are valid strings but 01010011, 01010, 101 are not. Examples:

Input : 100
Output : VALID

Input : 1110001
Output : NOT VALID
There is 0 between 1s

Input : 00011
Output : VALID

In Set 1, we have discussed general approach for validity of string.In this post, we will discuss regular expression approach for same and it is simple. As we know that in a string if there is zero between 1's, than string is not valid.Hence below is one of the regular expression for invalid string pattern.

10+1

So here is the simple regex algorithm.

1. Loop over the matcher(string)
2. if above regex match is find in the matcher, then string is not valid, otherwise valid.

Implementation:

VALID

Time complexity : O(n) 
Auxiliary Space : O(1)