Perl Cookbook

Finding the N^th Occurrence of a Match

Problem

You want to find the N ^th match in a string, not just the first one. For example, you’d like to find the word preceding the third occurrence of "fish":

               
                  One fish two fish red fish blue fish

Solution

Use the /g modifier in a while loop, keeping count of matches:

$WANT = 3;
$count = 0;
while (/(\w+)\s+fish\b/gi) {
    if (++$count == $WANT) {
        print "The third fish is a $1 one.\n";
        # Warning: don't `last' out of this loop
    }
}

                  The third fish is a red one.

Or use a repetition count and repeated pattern like this:

/(?:\w+\s+fish\s+){2}(\w+)\s+fish/i;

Discussion

As explained in the chapter introduction, using the /g modifier in scalar context creates something of a progressive match, useful in while loops. This is commonly used to count the number of times a pattern matches in a string:

# simple way with while loop
$count = 0;
while ($string =~ /PAT/g) {
    $count++;               # or whatever you'd like to do here
}

# same thing with trailing while
$count = 0;
$count++ while $string =~ /PAT/g;

# or with for loop
for ($count = 0; $string =~ /PAT/g; $count++) { }
    
# Similar, but this time count overlapping matches
$count++ while $string =~ /(?=PAT)/g;

To find the N^th match, it’s easiest to keep your own counter. When you reach the appropriate N, do whatever you care to. A similar technique could be used to find every N^th match by checking for multiples of N using the modulus operator. For example, (++$count % 3) == 0 would be every third match.

If this is too much bother, ...