The file is the README for Regexp::Assemble version 0.04

INSTALLATION

perl Makefile.PL
make
make test
make install

TESTING

This module requires the following modules for thorough testing:

    Test::Deep
    Test::File::Contents
    Test::More
    Test::Pod

I suspect I could rewrite the tests with out Test::Deep these days;
Test::More should be sufficient. This is mainly the result of legacy
architectural decisions that no longer hold true.

BASIC USAGE

use Regexp::Assemble;

my $ra = Regexp::Assemble->new;
$ra->add( 'ab+c' );
$ra->add( 'ab+\\d*\\s+c' );
$ra->add( 'a\\w+\\d+' );
$ra->add( 'a\\d+' );
print $ra->re; # prints (?:a(?:b+(?:\d*\s+)?c|(?:\w+)?\d+))

or

my $ra = Regexp::Assemble->new
    ->add( 'foo', 'bar', 'baz', 'foom' );

print "$_ matches\n" if /$ra/
    for (qw/word more stuff food rabble bark/);

or

use Regexp::Assemble;
my @word = qw/flip flop slip slop/;

print Regexp::Assemble->new->add(@word)->as_string;
    # produces [fs]l[io]p

print Regexp::Assemble->new->add(@word)->reduce(0)->as_string;
    # produces (?:fl(?:ip|op)|sl(?:ip|op))

See the ./eg directory for some example scripts. More will be added
in subsequent releases.

IMPLEMENTATION

Consider a simple pattern 'costructive' we want to use to match against
strings. This pattern is split into tokens, and is stored in a list:

  [c o n s t r u c t i v e]

At this point, if we want to produce a regular expression, we only need
to join it up again:

   my $pattern = join( '' => @path);
   my $re = qr/$pattern/;

Consider a second pattern 'containment'. Split into a list gives:

  [c o n t a i n m e n t]

We then have to merge this second path into the first path. At some
point, the paths diverge. The first element path the point of
divergence in the first path is replace by a node (a hash) and the two
different paths carry on from there:

  [c o n
        |s => [s t r u c t i v e]
        \t => [t a i n m e n t]
  ]

And then 'confinement':

  [c o n
        |s => [s t r u c t i v e]
        |t => [t a i n m e n t]
        \f => [f i n e m e n t]
  ]

What happens if we add a path that runs out in the middle of a
previous path?  We add a node, and a "null-path" to indicate that
the path can both continue on, and can also stop here:

Add 'construct':

  [c o n
        |s => [s t r u c t
        |                 | '' => undef
        |                 \ i => [i v e]
        |     ]
        |t => [t a i n m e n t]
        \f => [f i n e m e n t]
  ]

It should be obvious to see how the contruct branch will produce the
pattern /construct(?:ive)?/ . Or for a longer path 'constructively':

  [c o n
        |s => [s t r u c t
        |                 | '' => undef
        |                 \ i => [i v e
        |                              | '' => undef
        |                              \ l => [l y]
        |                        ]
        |     ]
        |t => [t a i n m e n t]
        \f => [f i n e m e n t]
  ]

This is the state of the internal structure before reduction. When
traversed it will produce a valid regular expression.

The trick is how to perform the reduction. The key insight is to note
that for any part of the trunk where the sibling paths do not end in
a node, it it possible to reverse them, and insert them into their
own R::A object and see what comes out:

  [t a i n m e n t] =>
  [t n e m n i a t]

  [f i n e m e n t] =>
  [t n e m e n i f]

Gives:

  [t n e m
          | n => [n i a t]
          \ e => [e n i f]
  ]

When the algorithm visits the other path (s => [s t r u c t ...]),
it behaves differently. When a null path is seen, no reduction is
performed at that node level. The resulting path would otherwise
begin to admit matches that are are not permitted by any of the
initial patterns. For instance, with bat, cat, and catty, you can
hardly try to merge 'bat' and 'cat' to produce [bc]at, otherwise the
resulting pattern would become [bc]at(ty)?, and that would incorrectly
match 'batty'.

After having visited the s, t, and f paths, the
result is that t and f were reduced, and s failed. We therefore
unreverse everything, and signal that this node cannot participate in
any more reduction (the failures percolate up the tree back to the
root).

Unreversing the t, f reduction gives:

  [ t => [t a i n] \
    f => [f i n e] | m e n t ]

When all is said and done, the final result gives

  [c o n
        |s => [s t r u c t
        |                 | '' => undef
        |                 \ i => [i v e
        |                              | '' => undef
        |                              \ l => [l y]
        |                        ]
        |     ]
        [ t => [t a i n]
          f => [f i n e] m e n t ]
  ]

When this data structure is traversed to build the pattern, it gives

  con(struct(ive(ly)?)?|(fine|tain)ment)

NB: The capturing syntax is used here, instead of the grouping
syntax for readability issues only.

On the other hand, if the s path contained only [s t r u c t], then
the reduction would have gone succeeded. We would have a common
head [t], shared by all three paths.

  [t
    | c => [c u r t s]
    \ n => [n e m
                 | n => [n i a t]
                 \ e => [e n i f]
           ]
  ]

And then consider that the path [c o u r t] had also been added to
the object. We would then be able to reduce the t from the above
reduction, and the t in [c o u r t]

  [c o
      | n => [n
      |        | s => [s t r u c t]
      |        | t => [t a i n m e n t]
      |        \ f => [f i n e m e n t]
      |      ]
      \ u => [u r t]
  ]

gives

  [c o
      | n => [n
      |        | s => [s t r u c]
      |        \ f => [
      |                 f => [f i n e]
      |                 t => [t a i n]
      |                 m e n
      |               ]
      |      ]
      \ u => [u r]
   t
  ]

(Here ends my ASCII art talents).

The above structure would give

  co(n(struc|(fine|tai)men)|ur)t

In a nutshell, that's it. Seems like the code would be simple, huh?
It turns out that no, there are lots of fiddly edge cases,
especially sets of paths are the same as other sets of paths
except for an optional sub-path. The canonical example that the
test suite deals with is:

  showeriness, showerless, showiness, showless.

The final pattern is

  show(er)?(in|l)ess

If there are bugs to be found, it will be in cases that are even
more pathological than this, e.g., something like:

  show(er)?(i(a|po)?n|l)ess

(although the above actually *does* work, I tried it)

This is the area that needs to be tested much more extensively.
Until now I haven't had the time (or motivation) to do so, mainly
because my real life patterns do not converge at the end very often.
On the other hand, I can say with a reasonable level of confidence
is in the case of a bug, the algorithm will splice a part of the
tree into oblivion. When this happens, part of the pattern will be
lost, and the resulting pattern will fail to match all that the
original patterns do. It will in no case ever match more things
than the original patterns do.

If you are truly paranoid, take a look at the hostmatch.t test
file. The code therein does exactly that: it takes a list of
patterns and a list of target strings. It assembles the patterns
and then loops through the target strings, checking to see that
the assembled pattern and one of the orignal patterns make the
same decision as to a target string. 

Or to put that more clearly: if the assembled pattern matches,
then one of the original patterns should also match. If the
assembled pattern doesn't match, then none of the original
patterns should match.

The two scripts assemble and assemble-check, supplied as examples,
can also help you play around with this process.

DEBUGGING NOTES

If you are curious, you can dump out the internal data struct with
the following:

  use Data::Dumper;
  $Data::Dumper::Terse     = 0;
  $Data::Dumper::Indent    = 0;
  $Data::Dumper::Quotekeys = 0;
  $Data::Dumper::Pair      = '=>';

  print Dumper($r->_path);

A more compact representation can also be obtained with

  print Regexp::Assemble::_dump($r->_path);

STATUS

This module is under active development.

AUTHOR

David Landgren <david@landgren.net>

I do appreciate getting e-mail, especially about Perl. Please keep in
mind that I get a lot of spam, and take drastic measures to reduce the
flow. One of the measures involves a gigantic regular expression that
contains many thousands of patterns that match hostnames of dynamic
dialup/residential/home IP addresses. That pattern is of course built
with this module.

It would be ironic if I rejected your mail coming from such an address.
Please use your ISPs outbound MX, or pay what it takes to get your
reverse DNS changed to something else.

COPYRIGHT

This module is copyright (C) David Landgren 2004. All rights reserved.

LICENSE

This library is free software; you can redistribute it and/or modify
it under the same terms as Perl itself.