How can I count the number of matches for a regex

Question

Let s say I have a string which contains this   HelloxxxHelloxxxHello   I compile a pattern to look for  Hello   Pattern pattern   Pattern compile  Hello    Matcher matcher   pattern matcher  HelloxxxHelloxxxHello      It should find three matches  How can I get a count of how many matches there were   I ve tried various loops and using the matcher groupCount   but it didn t work

User · Answer

If you want to use Java 8 streams and are allergic to while loops, you could try this:

public static int countPattern(String references, Pattern referencePattern) {
    Matcher matcher = referencePattern.matcher(references);
    return Stream.iterate(0, i -> i + 1)
            .filter(i -> !matcher.find())
            .findFirst()
            .get();
}

Disclaimer: this only works for disjoint matches.

Example:

public static void main(String[] args) throws ParseException {
    Pattern referencePattern = Pattern.compile("PASSENGER:\\d+");
    System.out.println(countPattern("[ \"PASSENGER:1\", \"PASSENGER:2\", \"AIR:1\", \"AIR:2\", \"FOP:2\" ]", referencePattern));
    System.out.println(countPattern("[ \"AIR:1\", \"AIR:2\", \"FOP:2\" ]", referencePattern));
    System.out.println(countPattern("[ \"AIR:1\", \"AIR:2\", \"FOP:2\", \"PASSENGER:1\" ]", referencePattern));
    System.out.println(countPattern("[  ]", referencePattern));
}

This prints out:

This is a solution for disjoint matches with streams:

public static int countPattern(String references, Pattern referencePattern) {
    return StreamSupport.stream(Spliterators.spliteratorUnknownSize(
            new Iterator<Integer>() {
                Matcher matcher = referencePattern.matcher(references);
                int from = 0;

                @Override
                public boolean hasNext() {
                    return matcher.find(from);
                }

                @Override
                public Integer next() {
                    from = matcher.start() + 1;
                    return 1;
                }
            },
            Spliterator.IMMUTABLE), false).reduce(0, (a, c) -> a + c);
}

User · Answer

Use the below code to find the count of number of matches that the regex finds in your input                     Pattern p   Pattern compile regex  Pattern MULTILINE   Pattern DOTALL      regex  here indicates your predefined regex          Matcher m   p matcher pattern       pattern  indicates your string to match the pattern against with         boolean b   m matches            if b          count            while  m find            count      This is a generalized code not specific one though  tailor it to suit your need  Please feel free to correct me if there is any mistake

User · Answer

matcher find   does not find all matches  only the next match   Solution for Java 9   long matches   matcher results   count      Solution for Java 8 and older  You ll have to do the following   Starting from Java 9  there is a nicer solution   int count   0  while  matcher find        count      Btw  matcher groupCount   is something completely different   Complete example   import java util regex     class Test       public static void main String   args            String hello    HelloxxxHelloxxxHello           Pattern pattern   Pattern compile  Hello            Matcher matcher   pattern matcher hello            int count   0          while  matcher find                count             System out println count         prints 3           Handling overlapping matches  When counting matches of aa in aaaa the above snippet will give you 2   aaaa aa   aa   To get 3 matches  i e  this behavior   aaaa aa  aa   aa   You have to search for a match at index  lt start of last match gt    1 as follows   String hello    aaaa   Pattern pattern   Pattern compile  aa    Matcher matcher   pattern matcher hello    int count   0  int i   0  while  matcher find i         count        i   matcher start     1     System out println count         prints 3

User · Answer

From Java 9  you can use the stream provided by Matcher results    long matches   matcher results   count

User · Answer

This should work for matches that might overlap   public static void main String   args        String input    aaaaaaaa       String regex    aa       Pattern pattern   Pattern compile regex       Matcher matcher   pattern matcher input       int from   0      int count   0      while matcher find from             count            from   matcher start     1            System out println count

[java] How can I count the number of matches for a regex?

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