Question

I have a set of elements of size about 100-200. Let a sample element be X.

Each of the elements is a set of strings (number of strings in such a set is between 1 and 4). X = {s1, s2, s3}

For a given input string (about 100 characters), say P, I want to test whether any of the X is present in the string.

X is present in P iff for all s belong to X, s is a substring of P.

The set of elements is available for pre-processing.

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I want this to be as fast as possible within Java. Possible approaches which do not fit my requirements:

• Checking whether all the strings s are substring of P seems like a costly operation
• Because s can be any substring of P (not necessarily a word), I cannot use a hash of words
• I cannot directly use regex as s1, s2, s3 can be present in any order and all of the strings need to be present as substring

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Right now my approach is to construct a huge regex out of each X with all possible permutations of the order of strings. Because number of elements in X <= 4, this is still feasible. It would be great if somebody can point me to a better (faster/more elegant) approach for the same.

Please note that the set of elements is available for pre-processing and I want the solution in java.

Answer 1

You can use regex directly:

Pattern regex = Pattern.compile(
    "^               # Anchor search to start of string\n" +
    "(?=.*s1)        # Check if string contains s1\n" +
    "(?=.*s2)        # Check if string contains s2\n" +
    "(?=.*s3)        # Check if string contains s3", 
    Pattern.DOTALL | Pattern.COMMENTS);
Matcher regexMatcher = regex.matcher(subjectString);
foundMatch = regexMatcher.find();

foundMatch is true if all three substrings are present in the string.

Note that you might need to escape your “needle strings” if they could contain regex metacharacters.