Perl Cookbook

Processing a String One Character at a Time

Problem

You want to process a string one character at a time.

Solution

Use split with a null pattern to break up the string into individual characters, or use unpack if you just want their ASCII values:

@array = split(//, $string);

@array = unpack("C*", $string);

Or extract each character in turn with a loop:

while (/(.)/g) { 	# . is never a newline here
        # do something with $1
    }

Discussion

As we said before, Perl’s fundamental unit is the string, not the character. Needing to process anything a character at a time is rare. Usually some kind of higher-level Perl operation, like pattern matching, solves the problem more easily. See, for example, Section 7.7, where a set of substitutions is used to find command-line arguments.

Splitting on a pattern that matches the empty string returns a list of the individual characters in the string. This is a convenient feature when done intentionally, but it’s easy to do unintentionally. For instance, /X*/ matches the empty string. Odds are you will find others when you don’t mean to.

Here’s an example that prints the characters used in the string "an apple a day“, sorted in ascending ASCII order:

%seen = ();
$string = "an apple a day";
foreach $byte (split //, $string) {
    $seen{$byte}++;
}
print "unique chars are: ", sort(keys %seen), "\n";

                  unique chars are:  adelnpy

These split and unpack solutions give you an array of characters to work with. If you don’t want an array, you can use a pattern match with the ...