zbmowrey/weighted-levenshtein

Weighted Levenshtein, Optimal String Alignment, and Damerau-Levenshtein distance for PHP 8.4+ — a port of infoscout/weighted-levenshtein.

v1.1.1 2026-05-15 21:07 UTC

Requires

• php: ^8.4

Requires (Dev)

• friendsofphp/php-cs-fixer: ^3
• phpstan/phpstan: ^2
• phpunit/phpunit: ^11

MIT

• Zach Mowrey

• levenshtein
• damerau
• edit-distance
• damerau-levenshtein
• string-similarity
• osa
• optimal-string-alignment
• fuzzy-matching

README

Weighted Levenshtein, Optimal String Alignment, and Damerau-Levenshtein edit-distance algorithms for PHP 8.4+. Pure PHP, strict types, no extensions.

PHP port of weighted-levenshtein by David Su / InfoScout. The algorithms, default behavior, and asymmetric cost semantics are preserved exactly; only the API has been reshaped to feel native in modern PHP.

Why weighted distance?

Most edit-distance libraries treat every insertion, deletion, substitution, or transposition as cost 1. That's fine for generic fuzzy-matching but often not what you actually want.

• OCR correction. Substituting 0 for O is a likely confusion; substituting X for O is not. Give the first a smaller cost.
• Typo correction. On a QWERTY keyboard, X and Z are neighbors; X and K are not. Or: humans transpose adjacent letters frequently, so transpositions should be cheap.
• Domain-specific noise. If your input pipeline tends to drop trailing whitespace or duplicate digits, weight those operations accordingly.

This library lets you specify a cost per character (for insert/delete) and per ordered pair (for substitute/transpose), then runs the appropriate dynamic-programming algorithm.

Installation

composer require zbmowrey/weighted-levenshtein

Requires PHP 8.4 or newer.

Quick start

use Zbmowrey\WeightedLevenshtein\Distance;

// Default (uniform) costs of 1.0 per operation.
echo Distance::levenshtein('kitten', 'sitting');             // 3
echo Distance::optimalStringAlignment('ca', 'ac');           // 1 (one transposition)
echo Distance::damerauLevenshtein('ab', 'bca');              // 2

Cost maps are immutable value objects. Build them with withCost():

use Zbmowrey\WeightedLevenshtein\CharCostMap;
use Zbmowrey\WeightedLevenshtein\Distance;

$insertCosts = CharCostMap::uniform()->withCost('D', 1.5);
echo Distance::levenshtein('BANANAS', 'BANDANAS', $insertCosts);  // 1.5

Full examples

These are the same examples as the original Python README, translated 1:1. Every snippet has a corresponding test in tests/Readme/.

use Zbmowrey\WeightedLevenshtein\CharCostMap;
use Zbmowrey\WeightedLevenshtein\CharPairCostMap;
use Zbmowrey\WeightedLevenshtein\Distance;

// --- Insertion cost ---
$insertCosts = CharCostMap::uniform()->withCost('D', 1.5);
echo Distance::levenshtein('BANANAS', 'BANDANAS', $insertCosts);
// 1.5

// --- Deletion cost ---
$deleteCosts = CharCostMap::uniform()->withCost('S', 0.5);
echo Distance::levenshtein('BANANAS', 'BANANA', $insertCosts, $deleteCosts);
// 0.5

// --- Substitution cost (asymmetric!) ---
$subs = CharPairCostMap::uniform()->withCost('H', 'B', 1.25);
echo Distance::levenshtein('HANANA', 'BANANA', null, null, $subs);
// 1.25

// The reverse direction is unweighted because we never set ('B','H').
echo Distance::levenshtein('BANANA', 'HANANA', null, null, $subs);
// 1.0

// Make the reverse direction match by setting the other ordered pair.
$subs = $subs->withCost('B', 'H', 1.25);
echo Distance::levenshtein('BANANA', 'HANANA', null, null, $subs);
// 1.25

// --- Transposition cost (Damerau-Levenshtein) ---
$transposes = CharPairCostMap::uniform()->withCost('A', 'B', 0.75);
echo Distance::damerauLevenshtein('ABNANA', 'BANANA', null, null, null, $transposes);
// 0.75

// Like substitution, transposition is also asymmetric.
echo Distance::damerauLevenshtein('BANANA', 'ABNANA', null, null, null, $transposes);
// 1.0

// Set the other direction to make it symmetric.
$transposes = $transposes->withCost('B', 'A', 0.75);
echo Distance::damerauLevenshtein('BANANA', 'ABNANA', null, null, null, $transposes);
// 0.75

Short aliases are available as free functions:

use function Zbmowrey\WeightedLevenshtein\{lev, osa, dam_lev};

echo lev('kitten', 'sitting');     // 3
echo osa('ca', 'ac');              // 1
echo dam_lev('ab', 'ba');          // 1

Built-in cost map presets

Two opinionated presets ship under Zbmowrey\WeightedLevenshtein\Presets\ for the two most common weighted-distance use cases. Both return immutable CharPairCostMap instances, so you can layer your own overrides with withCost().

OCR output (OcrConfusions)

use Zbmowrey\WeightedLevenshtein\Distance;
use Zbmowrey\WeightedLevenshtein\Presets\OcrConfusions;

$substitute = OcrConfusions::common();              // default cost 0.25
echo Distance::levenshtein('FOOD', 'F00D', null, null, $substitute);  // 0.5
echo Distance::levenshtein('Hello', 'He11o', null, null, $substitute); // 0.5

OcrConfusions::common(float $cost = 0.25) covers a curated list of high-confidence OCR confusions in both directions. The full set:

• Zero cluster: 0↔O↔o, 0↔D, O↔D, O↔Q
• One cluster: 1↔l↔I↔i (every pair)
• Seven cluster: 1↔7, 2↔7, 7↔T, 7↔Z↔z
• Other digit/digit: 2↔Z↔z, 3↔5, 3↔8
• Other digit/letter: 5↔S↔s, 8↔B, 6↔G, 6↔b, 9↔g, 9↔q
• Lowercase letter pairs: c↔e, n↔h, u↔v, m↔n, f↔t, r↔n
• Uppercase letter pairs: C↔G, E↔F, M↔N, P↔R, U↔V, V↔Y

The list is intentionally conservative — only pairs that confuse across most fonts are included. Font-specific or low-resolution-only confusions (e.g. 4↔A, 0↔6, n↔u) and multi-character confusions (rn↔m, cl↔d, vv↔w) are not registered. Layer them on with withCost() if your data needs them.

Pairs not in the list keep the default cost of 1.0. Layer your domain-specific tweaks with withCost():

$substitute = OcrConfusions::common()
    ->withCost('I', '1', 0.05)  // your OCR confuses I and 1 more strongly
    ->withCost('1', 'I', 0.05);

Human typos (QwertyKeyboard)

use Zbmowrey\WeightedLevenshtein\Distance;
use Zbmowrey\WeightedLevenshtein\Presets\QwertyKeyboard;

$substitute = QwertyKeyboard::substituteCosts();
$transpose  = QwertyKeyboard::transposeCosts();

echo Distance::damerauLevenshtein(
    'helo',
    'hwlo',
    null,
    null,
    $substitute,   // w is adjacent to e on the keyboard
    $transpose,
);
// 0.5

Costs are derived from the Euclidean distance between keys on a standard staggered US QWERTY layout. Orthogonal and close-diagonal neighbors get the adjacent cost (default 0.5); one-key-removed pairs get the near cost (default 0.75); everything else stays at 1.0. Both lowercase and uppercase letters are populated. Mixed-case and mixed-shift-state pairs (e.g. q/W, Q/1) are left at default — those errors are rare in practice.

Override the thresholds if 0.5/0.75 don't fit your data:

$strict = QwertyKeyboard::substituteCosts(adjacentCost: 0.2, nearCost: 0.5);

API reference

Zbmowrey\WeightedLevenshtein\Distance

Method	Description
Distance::levenshtein(string $a, string $b, ?CharCostMap $insertCosts = null, ?CharCostMap $deleteCosts = null, ?CharPairCostMap $substituteCosts = null): float	Wagner-Fischer Levenshtein distance.
Distance::optimalStringAlignment(string $a, string $b, ?CharCostMap $insertCosts = null, ?CharCostMap $deleteCosts = null, ?CharPairCostMap $substituteCosts = null, ?CharPairCostMap $transposeCosts = null): float	Wagner-Fischer with a single adjacent transposition check; substrings used in a transposition cannot also be edited.
Distance::damerauLevenshtein(string $a, string $b, ?CharCostMap $insertCosts = null, ?CharCostMap $deleteCosts = null, ?CharPairCostMap $substituteCosts = null, ?CharPairCostMap $transposeCosts = null): float	True Damerau-Levenshtein distance with arbitrary non-overlapping adjacent transpositions.

Zbmowrey\WeightedLevenshtein\CharCostMap

Immutable per-character cost map for insert/delete operations.

Method	Description
CharCostMap::uniform(float $defaultCost = 1.0): self	Construct a map where every character has cost $defaultCost.
withCost(string $char, float $cost): self	Return a new map with $cost for the single ASCII byte $char.
cost(string $char): float	Look up the cost for $char.

Zbmowrey\WeightedLevenshtein\CharPairCostMap

Immutable per-ordered-pair cost map for substitute/transpose operations.

Method	Description
CharPairCostMap::uniform(float $defaultCost = 1.0): self	Construct a map where every ordered pair has cost $defaultCost.
withCost(string $from, string $to, float $cost): self	Return a new map with $cost for the ordered pair ($from, $to).
cost(string $from, string $to): float	Look up the cost for the ordered pair.

Zbmowrey\WeightedLevenshtein\Presets\OcrConfusions

Method	Description
OcrConfusions::common(float $cost = 0.25): CharPairCostMap	Curated OCR confusion substitutions in both directions.

Zbmowrey\WeightedLevenshtein\Presets\QwertyKeyboard

Method	Description
QwertyKeyboard::substituteCosts(float $adjacentCost = 0.5, float $nearCost = 0.75): CharPairCostMap	Adjacency-weighted substitution cost map for ASCII letters and digits on a US QWERTY layout.
QwertyKeyboard::transposeCosts(float $adjacentCost = 0.5, float $nearCost = 0.75): CharPairCostMap	Adjacency-weighted transposition cost map using the same layout.

Free function aliases

In namespace Zbmowrey\WeightedLevenshtein:

• lev(...) — alias for Distance::levenshtein(...).
• osa(...) — alias for Distance::optimalStringAlignment(...).
• dam_lev(...) — alias for Distance::damerauLevenshtein(...).

Limitations

• ASCII only. Inputs must consist of bytes 0–127. Any byte ≥ 128 raises InvalidArgumentException. This matches the original library, whose internal arrays are indexed by ord() over a 128-wide alphabet.
• Case sensitive. 'a' and 'A' are distinct characters.
• Costs are asymmetric. CharPairCostMap is keyed by ordered pairs. Setting ('A', 'B') does not set ('B', 'A'). If you want symmetry, set both.

Performance

Pure PHP. Suitable for typical fuzzy-matching workloads in the range of low thousands of comparisons per second on strings of ~100 characters. If you need C-speed, use the original Python library, or PHP's built-in levenshtein() for the unweighted case. Two-row rolling buffer is used for plain Levenshtein; full DP matrices are used for OSA and Damerau-Levenshtein because both need O(m × n) state.

Contributing

Contributions welcome. Open an issue or PR. The full QA suite is composer qa (PHPUnit + PHPStan level max + PHP-CS-Fixer dry-run). All three must be green for a PR to land.

License

MIT. See LICENSE. PHP port copyright © 2026 Zach Mowrey. Original Python library copyright © 2016 InfoScout, distributed under the MIT License.