/ 
String-Approx-3.13
River stage one • 6 direct dependents • 7 total dependents
/ String::Approx

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 errorrs, speling errors, closely related vocabularies (colour color), genetic mutations (GAG ACT), abbreviations (McScot, MacScot).

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.

MATCH

use String::Approx 'amatch';

amatch("pattern") 
amatch("pattern", @inputs) 
amatch("pattern", [ modifiers ])
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%".

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.)

The starting and ending positions of matching can be changed from the beginning and end of the input(s) to some other positions by using the modifiers

"initial_position=24"
"final_position=42"

SUBSTITUTE

use String::Approx 'asubstitute';

asubstitute("pattern", "replacement")
asubstitute("pattern", "replacement", @inputs) 
asubstitute("pattern", "replacement", [ modifiers ])
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).

The starting and ending positions of substitution can be changed from the beginning and end of the input(s) to some other positions by using the modifiers

"initial_position=24"
"final_position=42"

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.

The starting and ending positions of indexing can be changed from the beginning and end of the input(s) to some other positions by using the modifiers

"initial_position=24"
"final_position=42"

SLICE

use String::Approx 'aindex';

($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 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 that the size of the match will very probably be something you did not expect (such as longer than the pattern). This may or may not be fixed in future releases.

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)

The starting and ending positions of slicing can be changed from the beginning and end of the input(s) to some other positions by using the modifiers

"initial_position=24"
"final_position=42"

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.

There's no backwards-scanning 'arindex'.

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 with valuable test cases and other feedback: Jared August, Steve A. Chervitz, Alberto Fontaneda, Dmitrij Frishman, Lars Gregersen, Kevin Greiner, Mike Hanafey, Ricky Houghton, Helmut Jarausch, Ben Kennedy, Mark Land, Sergey Novoselov, Andy Oram, Stefan Ram, Stewart Russell, Slaven Rezic, Chris Rosin, Ilya Sandler, Bob J.A. Schijvenaars, Greg Ward, 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>