Counting number of occurrences of a string inside another (Perl)

What is the fastest way to count the number of times a certain string appears in a bigger one? My best guess would be to replace all instances of that string with nothing, calculate the difference of lengths and divide by the length of the substring, but that seems rather slow, and I need to analyze big amounts of data.

Solution 1:

You can capture the strings, then count them. It can be done by applying a list context to the capture with ():

my $x = "foo";
my $y = "foo foo foo bar";
my $c = () = $y =~ /$x/g;  # $c is now 3

You can also capture to an array and count the array. Same principle, different technique:

my @c = $y =~ /$x/g;
my $count = @c;

Solution 2:

my $string = "aaaabbabbba";
my @count = ($string =~ /a/g);
print @count . "\n";

or

my $count = ($string =~ s/a/a/g);

Solution 3:

You could use a global regex. Something like:

my @matches = $bigstring =~ /($littlestring)/g;
my $count = @matches;