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Posted by PerlFAQ Server on February 27, 2008, 3:03 pm
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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 .
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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
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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