Get or alter a portion of a stirng
Extracts a substring out of EXPR and returns it. First character is at offset
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 LEN
is omitted, returns everything to the end of the string. If LEN is negative, leaves that
many characters off the end of the string.
If you specify a substring that is partly outside the string, the part within the string is returned. If the
substring is totally outside the string a warning is produced.
You can use the substr() function as an lvalue, in which case EXPR
must be an lvalue. If you assign something shorter than LEN, the string will shrink, and
if you assign something longer than LEN, 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().
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.