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Perl Language Reference Manual
by Larry Wall and others
Paperback (6"x9"), 724 pages
ISBN 9781906966027
RRP £29.95 ($39.95)

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Extracts a substring out of EXPR and returns it. First character is at offset 0, or whatever you've set $[ to (but don't do that). If OFFSET is negative (or more precisely, less than $[), starts that far from the end of the string. If LENGTH is omitted, returns everything to 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 3-arg 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:

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

Prior to Perl version 5.9.1, the result of using an lvalue multiple times was unspecified.

ISBN 9781906966027Perl Language Reference ManualSee the print edition