A substring of a string  is a contiguous sequence of the characters appearing in . The empty string is a substring of every string. A subsequence of a string  is a sequence of characters from , which may not be contiguous in . Every substring of  is a subsequence of . For example,  is a substring of , and  is a subsequence of  which is not a substring.

The LongestCommonSubString(s1, s2) command returns from its input strings, s1 and s2, a common substring of maximum length.

Many common substrings of maximum length may exist. Which among the candidates is returned depends upon the suffix structure of the pair of strings, but is deterministic.

The LongestCommonSubSequence(s1, s2) command is similar, but searches for subsequences of the pair of input strings rather than substrings.