Get or alter a portion of a stirng

Daftar Isi

• substr EXPR,OFFSET,LENGTH,REPLACEMENT

• substr EXPR,OFFSET,LENGTH
• substr EXPR,OFFSET

  Extracts a substring out of EXPR and returns it. First character is atoffset zero. If OFFSET is negative, startsthat far back from the end of the string. If LENGTH is omitted, returnseverything through the end of the string. If LENGTH is negative, leaves thatmany 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 EXPRmust 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 samelength, you may need to pad or chop your value using sprintf.

  If OFFSET and LENGTH specify a substring that is partly outside thestring, only the part within the string is returned. If the substringis beyond either end of the string, substr() returns the undefinedvalue and produces a warning. When used as an lvalue, specifying asubstring 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 thereplacement string as the 4th argument. This allows you to replaceparts 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 asa 'magic bullet'; each time it is assigned to, it remembers which partof 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
   
   }
   

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

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

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