oracular (3) String::Approx.3pm.gz
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.
MODIFIERS
With the modifiers you can control the amount of approximateness and certain other control variables.
The modifiers are one or more strings, for example "i", within a string optionally separated by
whitespace. The modifiers are inside an anonymous array: the [ ] in the syntax are not notational, they
really do mean [ ], for example [ "i", "2" ]. ["2 i"] would be identical.
The implicit default approximateness is 10%, rounded up. In other words: every tenth character in the
pattern may be an error, an edit. You can explicitly set the maximum approximateness by supplying a
modifier like
number
number%
Examples: "3", "15%".
Note that "0%" is not rounded up, it is equal to 0.
Using a similar syntax you can separately control the maximum number of insertions, deletions, and
substitutions by prefixing the numbers with I, D, or S, like this:
Inumber
Inumber%
Dnumber
Dnumber%
Snumber
Snumber%
Examples: "I2", "D20%", "S0".
You can ignore case ("A" becames equal to "a" and vice versa) by adding the "i" modifier.
For example
[ "i 25%", "S0" ]
means ignore case, allow every fourth character to be "an edit", but allow no substitutions. (See NOTES
about disallowing substitutions or insertions.)
NOTE: setting "I0 D0 S0" is not equivalent to using index(). If you want to use index(), use index().
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.
TEXT POSITION
The starting and ending positions of matching, substituting, indexing, or slicing can be changed from the
beginning and end of the input(s) to some other positions by using either or both of the modifiers
"initial_position=24"
"final_position=42"
or the both the modifiers
"initial_position=24"
"position_range=10"
By setting the "position_range" to be zero you can limit (anchor) the operation to happen only once (if a
match is possible) at the position.
VERSION
Major release 3.
CHANGES FROM VERSION 2
GOOD NEWS
The version 3 is 2-3 times faster than version 2
No pattern length limitation
The algorithm is independent on the pattern length: its time complexity is O(kn), where k is the
number of edits and n the length of the text (input). The preprocessing of the pattern will of
course take some O(m) (m being the pattern length) time, but amatch() and asubstitute() cache the
result of this preprocessing so that it is done only once per pattern.
BAD NEWS
You do need a C compiler to install the module
Perl's regular expressions are no more used; instead a faster and more scalable algorithm written in
C is used.
asubstitute() is now always stingy
The string matched and substituted is now always stingy, as short as possible. It used to be as long
as possible. This is an unfortunate change stemming from switching the matching algorithm. Example:
with edit distance of two and substituting for "word" from "cork" and "wool" previously did match
"cork" and "wool". Now it does match "or" and "wo". As little as possible, or, in other words, with
as much approximateness, as many edits, as possible. Because there is no need to match the "c" of
"cork", it is not matched.
no more aregex() because regular expressions are no more used
no more "compat1" for String::Approx version 1 compatibility
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.