Perl Programming/Keywords/substr

The substr keyword

The substr command extracts and returns a substring out of EXPRESSION, where the first character is at OFFSET 0. If OFFSET is negative, it starts to count from the end of the string. If LENGTH is negative, it leaves that much characters from the string's end.

substr can also be used as an lvalue, if EXPRESSION is also an lvalue. By an assignment larger or shorter than LENGTH, the string will grow or shrink respectively to accommodate it.

If OFFSET and LENGTH specify a substring partly outside the string, only the part within the string is returned. If it is beyond both ends, undef is returned, and an exception is raised, if it is an lvalue.

Syntax

  substr EXPRESSION, OFFSET, LENGTH, REPLACEMENT
  substr EXPRESSION, OFFSET, LENGTH
  substr EXPRESSION, OFFSET

Examples

The code

my $name = 'Anton';

print "name = \"", $name, "\"\n";
substr($name, 5) = 'io';         # $name is now "Antonio"

print "name = \"", $name, "\"\n";
my $null = substr $name, 7, 2;   # returns "" w/o warning

print "null = \"", $null, "\"\n";
my $oops = substr $name, 8;      # returns undefined with a warning

print "oops = \"", $oops, "\"\n";
substr($name, 8) = 'gap';        # raises an exception

print "name = \"", $name, "\"\n";

returns

name = "Anton"
name = "Antonio"
null = ""
oops = ""
substr outside of string at substr.pl line 13.