FAQ 4.29: How can I count the number of occurrences of a substring within a string?

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4.29: How can I count the number of occurrences of a substring within a string?

    There are a number of ways, with varying efficiency. If you want a count
    of a certain single character (X) within a string, you can use the
    "tr///" function like so:

        $string = "ThisXlineXhasXsomeXx'sXinXit";
        $count = ($string =~ tr/X//);
        print "There are $count X characters in the string";

    This is fine if you are just looking for a single character. However, if
    you are trying to count multiple character substrings within a larger
    string, "tr///" won't work. What you can do is wrap a while() loop
    around a global pattern match. For example, let's count negative

        $string = "-9 55 48 -2 23 -76 4 14 -44";
        while ($string =~ /-\d+/g) { $count++ }
        print "There are $count negative numbers in the string";

    Another version uses a global match in list context, then assigns the
    result to a scalar, producing a count of the number of matches.

            $count = () = $string =~ /-\d+/g;


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