Programming Perl, 4th Edition

unpack

unpack TEMPLATE, EXPR

This function does the reverse of pack: it expands a string (EXPR) representing a data structure into a list of values according to the TEMPLATE and returns those values. In scalar context, it can be used to unpack a single value. The TEMPLATE here has much the same format as it has in the pack function—it specifies the order and type of the values to be unpacked. See pack for a detailed description of TEMPLATE. An invalid element in the TEMPLATE, or an attempt to move outside the string with the x, X, or @ formats, raises an exception.

The string is broken into chunks described by the TEMPLATE. Each chunk is separately converted to a value. Typically, the bytes of the string either are the result of a pack or represent a C structure of some kind.

If the repeat count of a field is larger than the remainder of the input string allows, the repeat count is silently decreased. (Normally, you’d use a repeat count of * here, anyway.) If the input string is longer than what TEMPLATE describes, the rest of the string is ignored.

The unpack function is also useful for plain text data, not just binary data. Imagine that you had a data file that contained records that looked like this:

2009 The Graveyard Book Neil Gaiman 2008 The Yiddish Policemen’s Union Michael Chabon 2007 Rainbows End Vernor Vinge 2006 Spin Robert Charles Wilson 2005 Jonathan Strange & Mr Norrell Susanna Clarke 2004 Paladin of Souls Lois McMaster Bujold 2003 Hominids Robert J. Sawyer 2002 American ...