String::Expand - string utility functions for expanding variables in self-referential sets


 use String::Expand qw( expand_strings );

 my %vars = ( MESSAGE => 'My home is $HOME',
              TEXT    => 'Message is "$MESSAGE"' );

 expand_strings( \%vars, \%ENV );

 # %vars now contains something like:
 #   MESSAGE => 'My home is /home/user',
 #   TEXT    => 'Message is "My home is /home/user"'


This module implements utility functions for expanding embedded variables in a string. Variable references are embedded in strings in a similar form to the Bourne shell, namely, in the form $NAME or ${NAME}. In the former case, the NAME must consist of a capital letter or underscore, and may be followed by zero or more capital letters, digits or underscores. In the latter case, the name can consist of any characters, but will be terminated by the first close brace character '}'.

The string may also contain literal dollar marks, escaped by \$, and literal escape marks, escaped by \\. These will be converted to $ and \ respectively on return.

While there are many other modules that also provide expansion such as this, this module provides the function expand_strings(), which will perform variable expansions in all the values in a given hash, where values can refer to other values within the same hash.


$expanded = expand_string( $str, \%vars )

This function expands embedded variable references in the passed string, and returns the expanded copy.


A string possibly containing variable expansions


Reference to a hash containing variable values


A string with variables expanded

expand_strings( \%strs, \%overlay )

This function takes a hash of strings, and expands variable names embedded in any of them, in the same form as the string passed to expand_string(). Expansions may refer to other strings, or to values in the %overlay hash. Values in the main variables hash take precidence over values in the overlay.

Where values refer to other values, care must be taken to avoid cycles. If a cycle is detected while attempting to expand the values, then an exception is thrown.


Reference to a hash containing variables to expand


Reference to a hash containing other variable values





Paul Evans <>