Provided by: libstring-approx-perl_3.27-1build1_amd64 
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.