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List::Prefixed - Prefixed String List


  use List::Prefixed;

  # construct a new prefixed tree
  $folded = List::Prefixed->fold(qw( Fu Foo For Form Foot Food Ba Bar Baz ));

  # get all items sharing a common prefix
  @list = $folded->list('Fo'); # Foo, Food, Foot, For, Form

  # serialize as regular expression
  $regex = $folded->regex; # '(?:Ba(?:r|z)?|F(?:o(?:o(?:d|t)?|r(?:m)?)|u))'

  # de-serialize from regular expression
  $unfolded = List::Prefixed->unfold($regex);


The idea of a Prefixed List comes from regular expressions determining a finite list of words, like this:


The expression above matches exactly these strings:

  "Ba", "Bar", "Baz", "Foo", "Food", "Foot", "For", "Form", "Fu".

Representing a string list that way can have some advantages in certain situations:

  • The regular expression provides efficient test methods on arbitrary strings (e.g. whether or not a string is contained in the list or starts or ends with an element from the list).

  • The representaion is compressing, depending on how many shared prefixes appear in a list.

  • Conversely, a prefixed list can be efficiently set up from such a regular expression. Thus, the prefixed list leads to a natural way of serialization and de-serialization.

  • Sub lists sharing a common prefix can be extracted efficently from a prefixed list. This leads to an efficient implementation of auto-completion.

For example, from Perl package names indexed on CPAN, one can get a list of about 82K module names that takes more than 2M data. We can compress the list to a regular expression of about 900K that matches exactly all these names.

A Prefixed List is a tree consisting of node triples, formally defined as follows:

  node: ( prefix [node-list] opt )
      prefix: String
      node-list: List of node
      opt: Boolean

The list elements are the prefix strings, each of them appended to the prefix of the parent node. The opt flag is true if the list of sub nodes is optional, i.e., if the node prefix appended together with the parent prefixes is also contained in the list itself.

Any string list has a trivial representation that way, if one takes each string as the prefix of a node with empty node-list and collects all these nodes into a parent node with empty prefix.

A prefixed tree is called folded, if it's in minimal form, i.e. if there are no two child nodes in a parent node sharing a common left part in their prefixes. Obviously, for each string list, there exists a unique folded Prefixed Tree representation.



  $prefixed = List::Prefixed->new( @list );

This is an alias of the fold method.


  $prefixed = List::Prefixed->fold( @list );

Constructs a new folded List::Prefixed tree from the given string list.


  $prefixed = List::Prefixed->unfold( $regex );

Constructs a new List::Prefixed tree from a regular expression string. The string argument shuld be obtained from the regex method.


  @list = $prefixed->list;
  @list = $prefixed->list( $string );

Returns the list of list elements starting with the given string if a string argument is present or the whole list otherwise. In scalar context an ARRAY reference is returned.


  $regex = $prefixed->regex;

Returns a minimized regular expression (as string) matching exactly the strings the object has been constructed with.

You can control the escaping style of the expression. The default behavior is to apply Perl's quotemeta function and replace any non-ASCII character with \x{FFFF}, where FFFF is the hexadecimal character code. This is the Perl-compatible or PCRE style. To obtain an expression compatible with Java and the like, use

  use List::Prefixed uc_escape_style => 'Java'; # \uFFFF style

To skip Unicode escaping completely, use

  use List::Prefixed uc_escape_style => undef;  # do not escape

Alternatively, you can control the style at runtime by way of configuration variables.



Controls the escaping style for Unicode (non-ASCII) characters. The value can be one of the following:


Default style \x{FFFF}


Java etc. style \uFFFF


Do not escape Unicode characters at all. This may result in shorter expressions but may cause encoding issues under some circumstances.


By providing string functions one can customize the escaping behavior arbitrarily. In this case, $UC_ESCAPE_STYLE has no effect.


The term prefix refers to the storage order of characters. That is, prefix filtering with right-to-left written Unicode strings (such as Arabic or Hebrew) goes to the wrong direction from the user's point of view.

Large lists may cause deep recursion within the fold method. To avoid a lot of Deep recursion on anonymous subroutine warnings, there is a

  no warnings 'recursion'

directive in place. This is worth mentioning, though it's not actually a bug.


Strictly OO, exports nothing.



Sebastian Böthin, <>


Copyright (C) 2015 by Sebastian Böthin

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself, either Perl version 5.14.2 or, at your option, any later version of Perl 5 you may have available.