- substr
EXPR,OFFSET,LENGTH,REPLACEMENT
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- substr EXPR,OFFSET,LENGTH
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- substr EXPR,OFFSET
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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.
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 is a fatal error. 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'; # fatal error
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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().
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