Let’s begin with the syntax forms of this function:

index STR,SUBSTR,POSITION 
index STR,SUBSTR

• STR - the string you are searching
• SUBSTR - the substring whose position you want to locate or to search for
• POSITION – the position in the string from where you want to start the searching or the number of characters to skip over before searching for the substring

my $someText = "Learning Perl Language";
my $found = index $someText, "Perl";
my $found1 = index($someText, "perl", 5);
print "\$found = $found,\$found1 = $found1\n";
# it prints $found = 9,$found1 = - 1

my $found1 = index(uc $someText, "PERL", 5);
print "\$found1 = $found1\n";
# it prints $found1 = 9

Remember that Perl index function returns -1 if it doesn’t succeed; if you want to use it in a conditional test, you must test if the returning value is greater or equal to 0 for success:

if((my $pos = index($someText, "Perl")) >= 0)
{
  print "The text was located at the position $pos\n";
}

Or you can replace the first line of the code above with:

if(($pos = index($someText, "Perl")) > -1)

If you want to find out if a substring matches a bigger string and you don’t care about the substring position within the string, you can use a single line of code, like in the following example:

print "Found it!\n" if(index($someText,"Perl")>-1);

See the following example:

# initialize the original string
my $input = "Perl index, Perl chop, Perl hex";
my $pos = 0;

my @positions = ();

while (1) {
  $pos = index(lc $input, "perl", $pos);
  last if($pos < 0);
  push(@positions, $pos++);
}	

if (scalar (@positions) > 0) {
  print "substring found at positions: ", "@positions\n";
} else {
  print "substring not found\n";
}

If there will be a match of the substring in the scalar $input string, the index function will return the position of that occurrence in the scalar variable $pos, which next is added to @positions array by using Perl push function.

The loop will end when the index function will return a value less than zero – no occurrence was found. In order lo leave the while loop, we use the loop control keyword last.

Finally, we print @positions array which will display all the occurrences of the substring "Perl" in the $input string. Please note the using of Perl lc (lowercase) function in order to make our search case insensitive.

Running this snippet code will produce the following result:

substring found at positions: 0 12 23

my $input = '    Perl functions: trim, index';
my $substr = 'functions';
print "position without trim: ", index($input,$substr), "\n";
print "position with trim: ", index(trim($input),$substr), "\n";

# Declare the trim subroutine
sub trim
{
  my $string = shift;
  $string =~ s/^\s+//;            
  $string =~ s/\s+$//; 
  return $string;         
}

position without trim: 9
position with trim: 5

If you want to search for the last occurrence of a substring in a string, you need to use the built-in Perl rindex() function.

If you want to download the Perl index script with all the above examples included, please click here: Script download