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