NAME¶

String::Approx - Perl extension for approximate matching (fuzzy matching)

SYNOPSIS¶

  use String::Approx 'amatch';
  print if amatch("foobar");
  my @matches = amatch("xyzzy", @inputs);
  my @catches = amatch("plugh", ['2'], @inputs);

DESCRIPTION¶

String::Approx lets you match and substitute strings approximately. With this you can emulate errors: typing errors, spelling errors, closely related vocabularies (colour color), genetic mutations (GAG ACT), abbreviations (McScot, MacScot).

NOTE: String::Approx suits the task of string matching, not string comparison, and it works for strings, not for text.

If you want to compare strings for similarity, you probably just want the Levenshtein edit distance (explained below), the Text::Levenshtein and Text::LevenshteinXS modules in CPAN. See also Text::WagnerFischer and Text::PhraseDistance. (There are functions for this in String::Approx, e.g. adist(), but their results sometimes differ from the bare Levenshtein et al.)

If you want to compare things like text or source code, consisting of words or tokens and phrases and sentences, or expressions and statements, you should probably use some other tool than String::Approx, like for example the standard UNIX diff(1) tool, or the Algorithm::Diff module from CPAN.

The measure of approximateness is the Levenshtein edit distance. It is the total number of "edits": insertions,

        word world

deletions,

        monkey money

and substitutions

        sun fun

required to transform a string to another string. For example, to transform "lead" into "gold", you need three edits:

        lead gead goad gold

The edit distance of "lead" and "gold" is therefore three, or 75%.

String::Approx uses the Levenshtein edit distance as its measure, but String::Approx is not well-suited for comparing strings of different length, in other words, if you want a "fuzzy eq", see above. String::Approx is more like regular expressions or index(), it finds substrings that are close matches.>

MATCH¶

        use String::Approx 'amatch';
        $matched     = amatch("pattern") 
        $matched     = amatch("pattern", [ modifiers ])
        $any_matched = amatch("pattern", @inputs) 
        $any_matched = amatch("pattern", [ modifiers ], @inputs)
        @match       = amatch("pattern") 
        @match       = amatch("pattern", [ modifiers ])
        @matches     = amatch("pattern", @inputs) 
        @matches     = amatch("pattern", [ modifiers ], @inputs)

Match pattern approximately. In list context return the matched @inputs. If no inputs are given, match against the $_. In scalar context return true if any of the inputs match, false if none match.

Notice that the pattern is a string. Not a regular expression. None of the regular expression notations (^, ., *, and so on) work. They are characters just like the others. Note-on-note: some limited form of "regular expressionism" is planned in future: for example character classes ([abc]) and any-chars (.). But that feature will be turned on by a special modifier (just a guess: "r"), so there should be no backward compatibility problem.

Notice also that matching is not symmetric. The inputs are matched against the pattern, not the other way round. In other words: the pattern can be a substring, a submatch, of an input element. An input element is always a superstring of the pattern.

        number
        number%

        Inumber
        Inumber%
        Dnumber
        Dnumber%
        Snumber
        Snumber%

        [ "i 25%", "S0" ]

SUBSTITUTE¶

        use String::Approx 'asubstitute';
        @substituted = asubstitute("pattern", "replacement")
        @substituted = asubstitute("pattern", "replacement", @inputs) 
        @substituted = asubstitute("pattern", "replacement", [ modifiers ])
        @substituted = asubstitute("pattern", "replacement",
                                   [ modifiers ], @inputs)

Substitute approximate pattern with replacement and return as a list <copies> of @inputs, the substitutions having been made on the elements that did match the pattern. If no inputs are given, substitute in the $_. The replacement can contain magic strings $&, $`, $' that stand for the matched string, the string before it, and the string after it, respectively. All the other arguments are as in amatch(), plus one additional modifier, "g" which means substitute globally (all the matches in an element and not just the first one, as is the default).

See "BAD NEWS" about the unfortunate stinginess of asubstitute().

INDEX¶

        use String::Approx 'aindex';
        $index   = aindex("pattern")
        @indices = aindex("pattern", @inputs)
        $index   = aindex("pattern", [ modifiers ])
        @indices = aindex("pattern", [ modifiers ], @inputs)

Like amatch() but returns the index/indices at which the pattern matches approximately. In list context and if @inputs are used, returns a list of indices, one index for each input element. If there's no approximate match, -1 is returned as the index.

NOTE: if there is character repetition (e.g. "aa") either in the pattern or in the text, the returned index might start "too early". This is consistent with the goal of the module of matching "as early as possible", just like regular expressions (that there might be a "less approximate" match starting later is of somewhat irrelevant).

There's also backwards-scanning arindex().

SLICE¶

        use String::Approx 'aslice';
        ($index, $size)   = aslice("pattern")
        ([$i0, $s0], ...) = aslice("pattern", @inputs)
        ($index, $size)   = aslice("pattern", [ modifiers ])
        ([$i0, $s0], ...) = aslice("pattern", [ modifiers ], @inputs)

Like aindex() but returns also the size (length) of the match. If the match fails, returns an empty list (when matching against $_) or an empty anonymous list corresponding to the particular input.

NOTE: size of the match will very probably be something you did not expect (such as longer than the pattern, or a negative number). This may or may not be fixed in future releases. Also the beginning of the match may vary from the expected as with aindex(), see above.

If the modifier

        "minimal_distance"

is used, the minimal possible edit distance is returned as the third element:

        ($index, $size, $distance) = aslice("pattern", [ modifiers ])
        ([$i0, $s0, $d0], ...)     = aslice("pattern", [ modifiers ], @inputs)

DISTANCE¶

        use String::Approx 'adist';
        $dist = adist("pattern", $input);
        @dist = adist("pattern", @input);

Return the edit distance or distances between the pattern and the input or inputs. Zero edit distance means exact match. (Remember that the match can 'float' in the inputs, the match is a substring match.) If the pattern is longer than the input or inputs, the returned distance or distances is or are negative.

        use String::Approx 'adistr';
        $dist = adistr("pattern", $input);
        @dist = adistr("pattern", @inputs);

Return the relative edit distance or distances between the pattern and the input or inputs. Zero relative edit distance means exact match, one means completely different. (Remember that the match can 'float' in the inputs, the match is a substring match.) If the pattern is longer than the input or inputs, the returned distance or distances is or are negative.

You can use adist() or adistr() to sort the inputs according to their approximateness:

        my %d;
        @d{@inputs} = map { abs } adistr("pattern", @inputs);
        my @d = sort { $d{$a} <=> $d{$b} } @inputs;

Now @d contains the inputs, the most like "pattern" first.

CONTROLLING THE CACHE¶

"String::Approx" maintains a LU (least-used) cache that holds the 'matching engines' for each instance of a pattern+modifiers. The cache is intended to help the case where you match a small set of patterns against a large set of string. However, the more engines you cache the more you eat memory. If you have a lot of different patterns or if you have a lot of memory to burn, you may want to control the cache yourself. For example, allowing a larger cache consumes more memory but probably runs a little bit faster since the cache fills (and needs flushing) less often.

The cache has two parameters: max and purge. The first one is the maximum size of the cache and the second one is the cache flushing ratio: when the number of cache entries exceeds max, max times purge cache entries are flushed. The default values are 1000 and 0.75, respectively, which means that when the 1001st entry would be cached, 750 least used entries will be removed from the cache. To access the parameters you can use the calls

        $now_max = String::Approx::cache_max();
        String::Approx::cache_max($new_max);
        $now_purge = String::Approx::cache_purge();
        String::Approx::cache_purge($new_purge);
        $limit = String::Approx::cache_n_purge();

To be honest, there are actually two caches: the first one is used far the patterns with no modifiers, the second one for the patterns with pattern modifiers. Using the standard parameters you will therefore actually cache up to 2000 entries. The above calls control both caches for the same price.

To disable caching completely use

        String::Approx::cache_disable();

Note that this doesn't flush any possibly existing cache entries, to do that use

        String::Approx::cache_flush_all();

NOTES¶

Because matching is by substrings, not by whole strings, insertions and substitutions produce often very similar results: "abcde" matches "axbcde" either by insertion or substitution of "x".

The maximum edit distance is also the maximum number of edits. That is, the "I2" in

        amatch("abcd", ["I2"])

is useless because the maximum edit distance is (implicitly) 1. You may have meant to say

        amatch("abcd", ["2D1S1"])

or something like that.

If you want to simulate transposes

        feet fete

you need to allow at least edit distance of two because in terms of our edit primitives a transpose is first one deletion and then one insertion.

        "initial_position=24"
        "final_position=42"

        "initial_position=24"
        "position_range=10"

VERSION¶

Major release 3.

CHANGES FROM VERSION 2¶

ACKNOWLEDGEMENTS¶

The following people have provided valuable test cases, documentation clarifications, and other feedback:

Jared August, Arthur Bergman, Anirvan Chatterjee, Steve A. Chervitz, Aldo Calpini, David Curiel, Teun van den Dool, Alberto Fontaneda, Rob Fugina, Dmitrij Frishman, Lars Gregersen, Kevin Greiner, B. Elijah Griffin, Mike Hanafey, Mitch Helle, Ricky Houghton, 'idallen', Helmut Jarausch, Damian Keefe, Ben Kennedy, Craig Kelley, Franz Kirsch, Dag Kristian, Mark Land, J. D. Laub, John P. Linderman, Tim Maher, Juha Muilu, Sergey Novoselov, Andy Oram, Ji Y Park, Eric Promislow, Nikolaus Rath, Stefan Ram, Slaven Rezic, Dag Kristian Rognlien, Stewart Russell, Slaven Rezic, Chris Rosin, Pasha Sadri, Ilya Sandler, Bob J.A. Schijvenaars, Ross Smith, Frank Tobin, Greg Ward, Rich Williams, Rick Wise.

The matching algorithm was developed by Udi Manber, Sun Wu, and Burra Gopal in the Department of Computer Science, University of Arizona.

AUTHOR¶

Jarkko Hietaniemi <jhi@iki.fi>

COPYRIGHT AND LICENSE¶

Copyright 2001-2013 by Jarkko Hietaniemi

This library is free software; you can redistribute it and/or modify under either the terms of the Artistic License 2.0, or the GNU Library General Public License, Version 2. See the files Artistic and LGPL for more details.

Furthermore: no warranties or obligations of any kind are given, and the separate file COPYRIGHT must be included intact in all copies and derived materials.