Perl Cookbook

Accessing Substrings

Problem

You want to access or modify just a portion of a string, not the whole thing. For instance, you’ve read a fixed-width record and want to extract the individual fields.

Solution

The substr function lets you read from and write to bits of the string.

$value = substr($string, $offset, $count);
$value = substr($string, $offset);
    
substr($string, $offset, $count) = $newstring;
substr($string, $offset)         = $newtail;

The unpack function gives only read access, but is faster when you have many substrings to extract.

# get a 5-byte string, skip 3, then grab 2 8-byte strings, then the rest
($leading, $s1, $s2, $trailing) =
    unpack("A5 x3 A8 A8 A*", $data);

# split at five byte boundaries
@fivers = unpack("A5" x (length($string)/5), $string);

# chop string into individual characters
@chars  = unpack("A1" x length($string), $string);

Discussion

Unlike many other languages that represent strings as arrays of bytes (or characters), in Perl, strings are a basic data type. This means that you must use functions like unpack or substr to access individual characters or a portion of the string.

The offset argument to substr indicates the start of the substring you’re interested in, counting from the front if positive and from the end if negative. If offset is 0, the substring starts at the beginning. The count argument is the length of the substring.

$string = "This is what you have";
#         +012345678901234567890  Indexing forwards  (left to right)
#          109876543210987654321- Indexing backwards (right to left)
#           note that 0 means 10 or 20, etc. above

$first  = substr($string, 0, 1);  # "T"
$start  = substr($string, 5, 2);  # "is"
$rest   = substr($string, 13);    # "you have"
$last   = substr($string, -1);    # "e"
$end    = substr($string, -4);    # "have"
$piece  = substr($string, -8, 3); # "you"

You can do more than just look at parts of the string with substr; you can actually change them. That’s because substr is a particularly odd kind of function—an lvaluable one, that is, a function that may itself be assigned a value. (For the record, the others are vec, pos, and as of the 5.004 release, keys. If you squint, local and my can also be viewed as lvaluable functions.)

$string = "This is what you have";
print $string;

                  This is what you have
substr($string, 5, 2) = "wasn't"; # change "is" to "wasn't"

                  This wasn't what you have
substr($string, -12)  = "ondrous";# "This wasn't wondrous"

                  This wasn't wondrous
substr($string, 0, 1) = "";       # delete first character

                  his wasn't wondrous
substr($string, -10)  = "";       # delete last 10 characters

                  his wasn'

You can use the =~ operator and the s///, m//, or tr/// operators in conjunction with substr to make them affect only that portion of the string.

# you can test substrings with =~
if (substr($string, -10) =~ /pattern/) {
    	print "Pattern matches in last 10 characters\n";
}

# substitute "at" for "is", restricted to first five characters
substr($string, 0, 5) =~ s/is/at/g;

You can even swap values by using several substrs on each side of an assignment:

# exchange the first and last letters in a string
$a = "make a hat";
(substr($a,0,1), substr($a,-1)) = (substr($a,-1), substr($a,0,1));
print $a;

                  take a ham

Although unpack is not lvaluable, it is considerably faster than substr when you extract numerous values at once. It doesn’t directly support offsets as substr does. Instead, it uses lowercase "x" with a count to skip forward some number of bytes and an uppercase "X" with a count to skip backward some number of bytes.

# extract column with unpack
$a = "To be or not to be";
$b = unpack("x6 A6", $a);  # skip 6, grab 6
print $b;

                  or not

($b, $c) = unpack("x6 A2 X5 A2", $a); # forward 6, grab 2; backward 5, grab 2
print "$b\n$c\n";

                  or
               
                  be

Sometimes you prefer to think of your data as being cut up at specific columns. For example, you might want to place cuts right before positions 8, 14, 20, 26, and 30. Those are the column numbers where each field begins. Although you could calculate that the proper unpack format is "A7 A6 A6 A6 A4 A*“, this is too much mental strain for the virtuously lazy Perl programmer. Let Perl figure it out for you. Use the cut2fmt function below:

sub cut2fmt {
    my(@positions) = @_;
    my $template   = '';
    my $lastpos    = 1;
    foreach $place (@positions) {
        $template .= "A" . ($place - $lastpos) . " ";
        $lastpos   = $place;
    }
    $template .= "A*";
    return $template;
}

$fmt = cut2fmt(8, 14, 20, 26, 30);
print "$fmt\n";

                  A7 A6 A6 A6 A4 A*

The powerful unpack function goes far beyond mere text processing. It’s the gateway between text and binary data.

See Also

The unpack and substr functions in perlfunc(1) and Chapter 3 of Programming Perl; the cut2fmt subroutine of Section 1.18; the binary use of unpack in Section 8.18