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

Discussion in 'Perl Misc' started by PerlFAQ Server, Apr 22, 2011.

  1. This is an excerpt from the latest version perlfaq4.pod, which
    comes with the standard Perl distribution. These postings aim to
    reduce the number of repeated questions as well as allow the community
    to review and update the answers. The latest version of the complete
    perlfaq is at http://faq.perl.org .

    --------------------------------------------------------------------

    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
    integers:

    $string = "-9 55 48 -2 23 -76 4 14 -44";
    while ($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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    PerlFAQ Server, Apr 22, 2011
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