Shubham and Valid Substrings

Recently, Shubham learnt about strings. So, He decided to give problem to his friend Gaurav about strings. He gave him a string containing only lower-case English alphabets and wants him to find the length of longest valid substring. Longest valid substring of a string is the string whose alphabets are continuous, for example, 'abcd', 'jkl', 'xyz' are all valid substrings of the string "abcdhjkle