perl substr

substr EXPR,OFFSET,LENGTH,REPLACEMENT
substr EXPR,OFFSET,LENGTH
substr EXPR,OFFSET
Extracts a substring out of EXPR and returns it. First character
is at offset zero. If OFFSET is negative, starts that far back
from the end of the string. If LENGTH is omitted, returns
everything through the end of the string. If LENGTH is negative,
leaves that many characters off the end of the string.

my $s = "The black cat climbed the green tree";
my $color = substr $s, 4, 5; # black
my $middle = substr $s, 4, -11; # black cat climbed the
my $end = substr $s, 14; # climbed the green tree
my $tail = substr $s, -4; # tree
my $z = substr $s, -4, 2; # tr

You can use the "substr" function as an lvalue, in which case EXPR
must itself be an lvalue. If you assign something shorter than
LENGTH, the string will shrink, and if you assign something longer
than LENGTH, the string will grow to accommodate it. To keep the
string the same length, you may need to pad or chop your value
using "sprintf".

If OFFSET and LENGTH specify a substring that is partly outside
the string, only the part within the string is returned. If the
substring is beyond either end of the string, "substr" returns the
undefined value and produces a warning. When used as an lvalue,
specifying a substring that is entirely outside the string raises
an exception. Here's an example showing the behavior for boundary
cases:

my $name = 'fred';
substr($name, 4) = 'dy'; # $name is now 'freddy'
my $null = substr $name, 6, 2; # returns "" (no warning)
my $oops = substr $name, 7; # returns undef, with warning
substr($name, 7) = 'gap'; # raises an exception

An alternative to using "substr" as an lvalue is to specify the
replacement string as the 4th argument. This allows you to replace
parts of the EXPR and return what was there before in one
operation, just as you can with "splice".

my $s = "The black cat climbed the green tree";
my $z = substr $s, 14, 7, "jumped from"; # climbed
# $s is now "The black cat jumped from the green tree"

Note that the lvalue returned by the three-argument version of
"substr" acts as a 'magic bullet'; each time it is assigned to, it
remembers which part of the original string is being modified; for
example:

my $x = '1234';
for (substr($x,1,2)) {
$_ = 'a'; print $x," "; # prints 1a4
$_ = 'xyz'; print $x," "; # prints 1xyz4
$x = '56789';
$_ = 'pq'; print $x," "; # prints 5pq9
}

With negative offsets, it remembers its position from the end of
the string when the target string is modified:

my $x = '1234';
for (substr($x, -3, 2)) {
$_ = 'a'; print $x," "; # prints 1a4, as above
$x = 'abcdefg';
print $_," "; # prints f
}

Prior to Perl version 5.10, the result of using an lvalue multiple
times was unspecified. Prior to 5.16, the result with negative
offsets was unspecified.