There are some algorithms of exact substring searching (e.g. Knuth-Morris-Pratt, Boyer-Moore etc.) I want to explain one of them which is called Z algorithm in some sources.
Z-boxes and Z-values
Let’s consider the concept of Z-box. Take the string
S = "abcxxxabyyy". We have an internal part
"ab" in the string which repeats its prefix. The internal
"ab" is a Z-box. It has the beginning at the position with index 6 and the end in 7 (0-based). Z-boxes are substrings which match string prefixes with the same length. For the Z-box, let’s call the corresponding prefix the prototype (it’s my term but I think it’s OK to use it.)