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This document uses PHP Chinese website manual Release
(PHP 4 >= 4.0.1, PHP 5)
levenshtein — 计算两个字符串之间的编辑距离
$str1
, string $str2
)$str1
, string $str2
, int $cost_ins
, int $cost_rep
, int $cost_del
)
编辑距离,是指两个字串之间,通过替换、插入、删除等操作将字符串str1
转换成str2
所需要操作的最少字符数量。
该算法的复杂度是 O(m*n),其中 n 和 m 分别是str1
和str2
的长度 (当和算法复杂度为O(max(n,m)**3)的 similar_text() 相比时,此函数还是相当不错的,尽管仍然很耗时。)。
在最简单的形式中,该函数只以两个字符串作为参数,并计算通过插入、替换和删除等操作将str1
转换成str2
所需要的操作次数。
第二种变体将采用三个额外的参数来定义插入、替换和删除操作的次数。此变体比第一种更加通用和适应,但效率不高。
str1
求编辑距离中的其中一个字符串
str2
求编辑距离中的另一个字符串
cost_ins
定义插入次数
cost_rep
定义替换次数
cost_del
定义删除次数
此函数返回两个字符串参数之间的编辑距离,如果其中一个字符串参数长度大于限制的255个字符时,返回-1。
Example #1 levenshtein() 例子:
<?php
// 输入拼写错误的单词
$input = 'carrrot' ;
// 要检查的单词数组
$words = array( 'apple' , 'pineapple' , 'banana' , 'orange' ,
'radish' , 'carrot' , 'pea' , 'bean' , 'potato' );
// 目前没有找到最短距离
$shortest = - 1 ;
// 遍历单词来找到最接近的
foreach ( $words as $word ) {
// 计算输入单词与当前单词的距离
$lev = levenshtein ( $input , $word );
// 检查完全的匹配
if ( $lev == 0 ) {
// 最接近的单词是这个(完全匹配)
$closest = $word ;
$shortest = 0 ;
// 退出循环;我们已经找到一个完全的匹配
break;
}
// 如果此次距离比上次找到的要短
// 或者还没找到接近的单词
if ( $lev <= $shortest || $shortest < 0 ) {
// 设置最接近的匹配以及它的最短距离
$closest = $word ;
$shortest = $lev ;
}
}
echo "Input word: $input \n" ;
if ( $shortest == 0 ) {
echo "Exact match found: $closest \n" ;
} else {
echo "Did you mean: $closest ?\n" ;
}
?>
以上例程会输出:
Input word: carrrot Did you mean: carrot?
[#1] qbolec [2015-08-05 20:10:02]
For the rare occassions, where you want:
1. multibyte UTF-8 characters
2. linear memory consumption (that is O(n+m) , not O(n*m))
3. learn the string which is the longest common subsequence
4. reasonable (that is O(n*m)) time complexity
Consider this implementation:
<?php
class Strings
{
public static function len($a){
return mb_strlen($a,'UTF-8');
}
public static function substr($a,$x,$y=null){
if($y===NULL){
$y=self::len($a);
}
return mb_substr($a,$x,$y,'UTF-8');
}
public static function letters($a){
$len = self::len($a);
if($len==0){
return array();
}else if($len == 1){
return array($a);
}else{
return Arrays::concat(
self::letters(self::substr($a,0,$len>>1)),
self::letters(self::substr($a,$len>>1))
);
}
}
private static function lcs_last_column(array $A,array $B){
$al=count($A);
$bl=count($B);
$last_column = array();
for($i=0;$i<=$al;++$i){
$current_row = array();
for($j=0;$j<=$bl;++$j){
//$a[0,$i) vs $b[0,$j)
if($i==0 || $j == 0){
$v = 0;
}else if($A[$i-1]===$B[$j-1]){
$v = 1 + $last_row[$j-1];
}else{
$v = max($last_row[$j],$current_row[$j-1]);
}
$current_row[] = $v;
}
$last_column[] = $current_row[$bl];
$last_row = $current_row;
}
return $last_column;
}
public static function lcs($a,$b){
$A = self::letters($a);
$B = self::letters($b);
$bl=count($B);
if($bl==0){
return '';
}else if($bl==1){
return FALSE===array_search($B[0],$A,true)?'':$B[0];
}
$left=self::lcs_last_column($A,array_slice($B,0,$bl>>1));
$right=array_reverse(self::lcs_last_column(array_reverse($A),array_reverse(array_slice($B,$bl>>1))));
$best_i = 0;
$best_lcs = 0;
foreach($left as $i => $lcs_left){
$option = $lcs_left + $right[$i];
if($best_lcs < $option){
$best_lcs = $option;
$best_i = $i;
}
}
return
self::lcs(self::substr($a,0,$best_i), self::substr($b,0,$bl>>1)).
self::lcs(self::substr($a,$best_i), self::substr($b,$bl>>1));
}
?>
This is a classic implentation in which several tricks are used:
1. the strings are exploded into multi-byte characters in O(n lg n) time
2. instead of searching for the longest path in a precomputed two-dimensional array, we search for the best point which lays in the middle column. This is achieved by spliting the second string in half, and recursively calling the algorithm twice. The only thing we need from the recursive call are the values in the middle column. The trick is to return the last column from each recursive call, which is what we need for the left part, but requires one more trick for the right part - we simply mirror the strings and the array so that the last column is the first column. Then we just find the row which maximizes the sum of lenghts in each part.
3. one can prove that the time consumed by the algorithm is proportional to the area of the (imaginary) two-dimensional array, thus it is O(n*m).
[#2] WiLDRAGoN [2015-07-15 08:19:20]
Some small changes allow you to calculate multiple words.
<?php
$input = array();
$dictionary = array();
foreach ($input as $output) {
$shortest = -1;
foreach ($dictionary as $word) {
$lev = levenshtein($output, $word);
if ($lev == 0) {
$closest = $word;
$shortest = 0;
}
if ($lev <= $shortest || $shortest < 0) {
$closest = $word;
$shortest = $lev;
}
}
echo "Input word: $output\n";
if ($shortest == 0) {
echo "Exact match found: $closest\n";
} else {
echo "Did you mean: $closest?\n";
}
}
?>
[#3] kars at kargn dot as [2014-03-28 02:13:03]
This is useful to detect spam user.
[#4] luciole75w at no dot spam dot gmail dot com [2013-11-18 14:25:00]
The levenshtein function processes each byte of the input string individually. Then for multibyte encodings, such as UTF-8, it may give misleading results.
Example with a french accented word :
- levenshtein('notre', 'votre') = 1
- levenshtein('notre', 'n?tre') = 2 (huh ?!)
You can easily find a multibyte compliant PHP implementation of the levenshtein function but it will be of course much slower than the C implementation.
Another option is to convert the strings to a single-byte (lossless) encoding so that they can feed the fast core levenshtein function.
Here is the conversion function I used with a search engine storing UTF-8 strings, and a quick benchmark. I hope it will help.
<?php
// Convert an UTF-8 encoded string to a single-byte string suitable for
// functions such as levenshtein.
//
// The function simply uses (and updates) a tailored dynamic encoding
// (in/out map parameter) where non-ascii characters are remapped to
// the range [128-255] in order of appearance.
//
// Thus it supports up to 128 different multibyte code points max over
// the whole set of strings sharing this encoding.
//
function utf8_to_extended_ascii($str, &$map)
{
// find all multibyte characters (cf. utf-8 encoding specs)
$matches = array();
if (!preg_match_all('/[\xC0-\xF7][\x80-\xBF]+/', $str, $matches))
return $str; // plain ascii string
// update the encoding map with the characters not already met
foreach ($matches[0] as $mbc)
if (!isset($map[$mbc]))
$map[$mbc] = chr(128 + count($map));
// finally remap non-ascii characters
return strtr($str, $map);
}
// Didactic example showing the usage of the previous conversion function but,
// for better performance, in a real application with a single input string
// matched against many strings from a database, you will probably want to
// pre-encode the input only once.
//
function levenshtein_utf8($s1, $s2)
{
$charMap = array();
$s1 = utf8_to_extended_ascii($s1, $charMap);
$s2 = utf8_to_extended_ascii($s2, $charMap);
return levenshtein($s1, $s2);
}
?>
Results (for about 6000 calls)
- reference time core C function (single-byte) : 30 ms
- utf8 to ext-ascii conversion + core function : 90 ms
- full php implementation : 3000 ms
[#5] engineglue at gmail dot com [2012-01-08 08:05:07]
I really like [the manual's] example for the use of the levenshtein function to match against an array. I ran into the need to specify the sensitivity of the result. There are circumstances when you want it to return false if the match is way out of line. I wouldn't want "marry had a little lamb" to match with "saw viii" simply because it was the best match in the array. Hence the need for sensitivity:
<?php
function wordMatch($words, $input, $sensitivity){
$shortest = -1;
foreach ($words as $word) {
$lev = levenshtein($input, $word);
if ($lev == 0) {
$closest = $word;
$shortest = 0;
break;
}
if ($lev <= $shortest || $shortest < 0) {
$closest = $word;
$shortest = $lev;
}
}
if($shortest <= $sensitivity){
return $closest;
} else {
return 0;
}
}
$word = 'PINEEEEAPPLE';
$words = array('apple','pineapple','banana','orange',
'radish','carrot','pea','bean','potato');
echo wordMatch($words, strtolower($word), 2);
?>
[#6] Chaim Chaikin [2011-11-02 11:40:20]
As regards to Example #1 above, would it not be more efficient to first use a simple php == comparison to check if the strings are equal even before testing the word with levenshtein().
Something like this:
<?php
// input misspelled word
$input = 'carrrot';
// array of words to check against
$words = array('apple','pineapple','banana','orange',
'radish','carrot','pea','bean','potato');
// no shortest distance found, yet
$shortest = -1;
// loop through words to find the closest
foreach ($words as $word) {
// check for an exact match
if ($input == $word) {
// closest word is this one (exact match)
$closest = $word;
$shortest = 0;
// break out of the loop; we've found an exact match
break;
}
// calculate the distance between the input word,
// and the current word
$lev = levenshtein($input, $word);
// if this distance is less than the next found shortest
// distance, OR if a next shortest word has not yet been found
if ($lev <= $shortest || $shortest < 0) {
// set the closest match, and shortest distance
$closest = $word;
$shortest = $lev;
}
}
echo "Input word: $input\n";
if ($shortest == 0) {
echo "Exact match found: $closest\n";
} else {
echo "Did you mean: $closest?\n";
}
?>
[#7] paulrowe at iname dot com [2008-08-27 17:58:19]
[EDITOR'S NOTE: original post and 2 corrections combined into 1 -- mgf]
Here is an implementation of the Levenshtein Distance calculation that only uses a one-dimensional array and doesn't have a limit to the string length. This implementation was inspired by maze generation algorithms that also use only one-dimensional arrays.
I have tested this function with two 532-character strings and it completed in 0.6-0.8 seconds.
<?php
function LevenshteinDistance($s1, $s2)
{
$sLeft = (strlen($s1) > strlen($s2)) ? $s1 : $s2;
$sRight = (strlen($s1) > strlen($s2)) ? $s2 : $s1;
$nLeftLength = strlen($sLeft);
$nRightLength = strlen($sRight);
if ($nLeftLength == 0)
return $nRightLength;
else if ($nRightLength == 0)
return $nLeftLength;
else if ($sLeft === $sRight)
return 0;
else if (($nLeftLength < $nRightLength) && (strpos($sRight, $sLeft) !== FALSE))
return $nRightLength - $nLeftLength;
else if (($nRightLength < $nLeftLength) && (strpos($sLeft, $sRight) !== FALSE))
return $nLeftLength - $nRightLength;
else {
$nsDistance = range(1, $nRightLength + 1);
for ($nLeftPos = 1; $nLeftPos <= $nLeftLength; ++$nLeftPos)
{
$cLeft = $sLeft[$nLeftPos - 1];
$nDiagonal = $nLeftPos - 1;
$nsDistance[0] = $nLeftPos;
for ($nRightPos = 1; $nRightPos <= $nRightLength; ++$nRightPos)
{
$cRight = $sRight[$nRightPos - 1];
$nCost = ($cRight == $cLeft) ? 0 : 1;
$nNewDiagonal = $nsDistance[$nRightPos];
$nsDistance[$nRightPos] =
min($nsDistance[$nRightPos] + 1,
$nsDistance[$nRightPos - 1] + 1,
$nDiagonal + $nCost);
$nDiagonal = $nNewDiagonal;
}
}
return $nsDistance[$nRightLength];
}
}
?>
[#8] luka8088 at gmail dot com [2008-05-28 13:03:50]
Simple levenshtein function without string length limit ...
<?php
function levenshtein2($str1, $str2, $cost_ins = null, $cost_rep = null, $cost_del = null) {
$d = array_fill(0, strlen($str1) + 1, array_fill(0, strlen($str2) + 1, 0));
$ret = 0;
for ($i = 1; $i < strlen($str1) + 1; $i++)
$d[$i][0] = $i;
for ($j = 1; $j < strlen($str2) + 1; $j++)
$d[0][$j] = $j;
for ($i = 1; $i < strlen($str1) + 1; $i++)
for ($j = 1; $j < strlen($str2) + 1; $j++) {
$c = 1;
if ($str1{$i - 1} == $str2{$j - 1})
$c = 0;
$d[$i][$j] = min($d[$i - 1][$j] + 1, $d[$i][$j - 1] + 1, $d[$i - 1][$j - 1] + $c);
$ret = $d[$i][$j];
}
return $ret;
}
?>
[#9] atx dot antrax at gmail dot com [2008-04-17 14:42:06]
I have made a function that removes the length-limit of levenshtein function and ajust the result with similar_text:
<?php
function _similar($str1, $str2) {
$strlen1=strlen($str1);
$strlen2=strlen($str2);
$max=max($strlen1, $strlen2);
$splitSize=250;
if($max>$splitSize)
{
$lev=0;
for($cont=0;$cont<$max;$cont+=$splitSize)
{
if($strlen1<=$cont || $strlen2<=$cont)
{
$lev=$lev/($max/min($strlen1,$strlen2));
break;
}
$lev+=levenshtein(substr($str1,$cont,$splitSize), substr($str2,$cont,$splitSize));
}
}
else
$lev=levenshtein($str1, $str2);
$porcentage= -100*$lev/$max+100;
if($porcentage>75)//Ajustar con similar_text
similar_text($str1,$str2,$porcentage);
return $porcentage;
}
?>
[#10] dale3h [2008-02-12 08:52:41]
Using PHP's example along with Patrick's comparison percentage function, I have come up with a function that returns the closest word from an array, and assigns the percentage to a referenced variable:
<?php
function closest_word($input, $words, &$percent = null) {
$shortest = -1;
foreach ($words as $word) {
$lev = levenshtein($input, $word);
if ($lev == 0) {
$closest = $word;
$shortest = 0;
break;
}
if ($lev <= $shortest || $shortest < 0) {
$closest = $word;
$shortest = $lev;
}
}
$percent = 1 - levenshtein($input, $closest) / max(strlen($input), strlen($closest));
return $closest;
}
?>
Usage:
<?php
$input = 'carrrot';
$words = array('apple','pineapple','banana','orange',
'radish','carrot','pea','bean','potato');
$percent = null;
$found = closest_word($input, $words, $percent);
printf('Closest word to "%s": %s (%s%% match)', $input, $found, round($percent * 100, 2));
?>
I found that lowercasing the array prior to comparing yields a better comparison when the case is not of importance, for example: comparing a user-inputted category to a list of existing categories.
I also found that when the percentage was above 75%, it was usually the match that I was looking for.
[#11] carey at NOSPAM dot internode dot net dot au [2006-10-28 07:06:52]
I have found that levenshtein is actually case-sensitive (in PHP 4.4.2 at least).
<?php
$distance=levenshtein('hello','ELLO');
echo "$distance";
?>
Outputs: "5", instead of "1". If you are implementing a fuzzy search feature that makes use of levenshtein, you will probably need to find a way to work around this.
[#12] dinesh AT dinsoft DOT net [2006-03-17 15:18:15]
Here is a string resynch function:
<?php
// Trouve les operations a effectuer pour modifier $b en $a en exploitant leurs similitudes (Finds the operations required to change $b to $a)
// Identique a la fonction Resynch Compare de Hex Workshop
//
// Parametres:
// $a Premiere chaine (cible, target)
// $b Seconde chaine (source)
// $l Nombre d'octets devant correspondre pour etre consides comme un bloc similaire (number of matching bytes required)
// $s Distance maximale dans laquelle les blocs similaires sont cherches (search window)
//
// Retourne:
// Array
// Array
// [0] Operation: + Add , - Del , / Replace, = Match
// [1] Source offset
// [2] Source count
// [3] Target offset
// [4] Target count
//
function str_resynch($a,$b,$l=32,$s=2048) {
$r=array();
for($i=0,$c=strlen($a),$cc=strlen($b),$ii=0,$z=$s-1,$z2=($z<<1)+1; $i<$c; $i++) {
$d=$i-$z;
$d=($d<$ii)?substr($b,$ii,$z2-$ii+$d):substr($b,$d,$z2);
$p=strpos($d,$a{$i});
$n=0;
while ($p!==FALSE) {
$m=1;
$bi=$i;
$bp=$p;
$p+=$ii;
while ((++$i<$c) && (++$p<$cc)) {
if ($a{$i}!=$b{$p}) break;
$m++;
}
if ($m<$l) {
$i=$bi;
$n=$bp+1;
$p=@strpos($d,$a{$i},$n);
}
else {
$i--;
$r[]=array($bi,$bp+$ii,$m); // offset a, offset b, Count
$ii=$p;
break;
}
}
}
if (!count($r)) return ($cc)?array('/',0,$c,0,$cc):array(array('+',0,$c,0,0));
$o=array();
$bi=0;
$bp=0;
for($i=0,$m=count($r);$i<$m;$i++) {
if ($r[$i][0]!=$bi) {
if ($r[$i][1]!=$bp) {
// Replace
$o[]=array('/',$bi,$r[$i][0]-$bi,$bp,$r[$i][1]-$bp);
$bi=$r[$i][0];
$bp=$r[$i][1];
}
else {
// Insertion
$o[]=array('+',$bi,$r[$i][0]-$bi,$bp,0);
$bi=$r[$i][0];
}
}
elseif ($r[$i][1]!=$bp) {
// Delete
$o[]=array('-',$bi,0,$bp,$r[$i][1]-$bp);
$bp=$r[$i][1];
}
// Match
$o[]=array('=',$r[$i][0],$r[$i][2],$r[$i][1],$r[$i][2]);
$bi+=$r[$i][2];
$bp+=$r[$i][2];
}
if ($c!=$bi) {
if ($cc!=$bp) $o[]=array('/',$bi,$c-$bi,$bp,$cc-$bp);
else $o[]=array('+',$bi,$c-$bi,$bp,0);
}
elseif ($cc!=$bp) $o[]=array('-',$bi,0,$bp,$cc-$bp);
return $o;
}
?>
[#13] june05 at tilo-hauke dot de [2005-06-06 00:44:55]
//levenshtein for arrays
function array_levenshtein($array1,$array2)
{ $aliases= array_flip(array_values(array_unique(array_merge($array1,$array2))));
if(count($aliases)>255) return -1;
$stringA=''; $stringB='';
foreach($array1 as $entry) $stringA.=chr($aliases[$entry]);
foreach($array2 as $entry) $stringB.=chr($aliases[$entry]);
return levenshtein($stringA,$stringB);
}
// e.g. use array_levenshtein to detect special expressions in unser-inputs
echo array_levenshtein(split(" ", "my name is xxx"), split(" ","my name is levenshtein"));
//output: 1
[#14] justin at visunet dot ie [2005-04-05 07:46:44]
<?php
function btlfsa($word1,$word2)
{
$score = 0;
// For each char that is different add 2 to the score
// as this is a BIG difference
$remainder = preg_replace("/[".preg_replace("/[^A-Za-z0-9\']/",' ',$word1)."]/i",'',$word2);
$remainder .= preg_replace("/[".preg_replace("/[^A-Za-z0-9\']/",' ',$word2)."]/i",'',$word1);
$score = strlen($remainder)*2;
// Take the difference in string length and add it to the score
$w1_len = strlen($word1);
$w2_len = strlen($word2);
$score += $w1_len > $w2_len ? $w1_len - $w2_len : $w2_len - $w1_len;
// Calculate how many letters are in different locations
// And add it to the score i.e.
//
// h e a r t
// 1 2 3 4 5
//
// h a e r t a e = 2
// 1 2 3 4 5 1 2 3 4 5
//
$w1 = $w1_len > $w2_len ? $word1 : $word2;
$w2 = $w1_len > $w2_len ? $word2 : $word1;
for($i=0; $i < strlen($w1); $i++)
{
if ( !isset($w2[$i]) || $w1[$i] != $w2[$i] )
{
$score++;
}
}
return $score;
}
// *************************************************************
// Here is a full code example showing the difference
$misspelled = 'haert';
// Imagine that these are sample suggestions thrown back by soundex or metaphone..
$suggestions = array('herat', 'haart', 'heart', 'harte');
// Firstly order an array based on levenshtein
$levenshtein_ordered = array();
foreach ( $suggestions as $suggestion )
{
$levenshtein_ordered[$suggestion] = levenshtein($misspelled,$suggestion);
}
asort($levenshtein_ordered, SORT_NUMERIC );
print "<b>Suggestions as ordered by levenshtein...</b><ul><pre>";
print_r($levenshtein_ordered);
print "</pre></ul>";
// Secondly order an array based on btlfsa
$btlfsa_ordered = array();
foreach ( $suggestions as $suggestion )
{
$btlfsa_ordered[$suggestion] = btlfsa($misspelled,$suggestion);
}
asort($btlfsa_ordered, SORT_NUMERIC );
print "<b>Suggestions as ordered by btlfsa...</b><ul><pre>";
print_r($btlfsa_ordered);
print "</pre></ul>";
?>
[#15] mcreuzer at r-world dot com [2005-03-07 09:01:47]
I am using the Levenshtein distance to SORT my search results.
I have a search page for peoples names. I do a SOUNDEX() search on the name in mysql. MySQL SOUNDEX() will perform the "fuzzy" search for me.
I then calculate the Levenshtein distance between the search term and the actual name found by the SOUNDEX() search. This will give me a score on how close my results are to the search string.
I can the sort my results for display listing the closest results first.
<?php
// PHP CODE INCLUDING DB LOOKUPS HERE
usort($searchresults, "finallevenshteinsortfunction");
function finallevenshteinsortfunction($a, $b)
{
if(($a['levenshtein'] > $b['levenshtein']) || ( $a['levenshtein'] == $b['levenshtein'] && strnatcasecmp( $a['Last_Name'], $b['Last_Name']) >= 1) ){ return $a['levenshtein'];} // Ok... The levenstein is greater OR with the same levenshtein, the last name is alphanumerically first
elseif($a['levenshtein'] == $b['levenshtein']){ return '0';} // The levenstein matches
elseif($a['levenshtein'] < $b['levenshtein']){ return -$a['levenshtein'];}
else{die("<!-- a horrable death -->");}
}
?>
[#16] gzink at zinkconsulting dot com [2003-12-03 03:03:09]
Try combining this with metaphone() for a truly amazing fuzzy search function. Play with it a bit, the results can be plain scary (users thinking the computer is almost telepathic) when implemented properly. I wish spell checkers worked as well as the code I've written.
I would release my complete code if reasonable, but it's not, due to copyright issues. I just hope that somebody can learn from this little tip!
[#17] genialbrainmachine at dot IHATESPAM dot tiscali dot it [2003-10-26 11:55:37]
I wrote this function to have an "intelligent" comparison between data to be written in a DB
and already existent data. Not ony calculating distances but also balancing distances for
each field.
<?php
function search_similar($record, $weights, $compared, $precision=2) {
$field_names = array_keys($record);
# "Weighted length" of $record and "weighted distance".
foreach ($field_names as $field_key) {
$record_weight += strlen($record[$field_key]) * $weights[$field_key];
$weighted_distance += levenshtein($record[$field_key],$compared[$field_key]) * $weights[$field_key];
}
# Building the result..
if ($record_weight) {
return round(($weighted_distance / $record_weight * 100),$precision);
} elseif ((strlen(implode("",$record)) == 0) && (strlen(implode("",$compared)) == 0)) { // empty records
return round(0,$precision);
} elseif (array_sum($weights) == 0) { // all weights == 0
return round(0,$precision);
} else {
return false;
}
}
?>
[#18] jlsalinas at gmx dot net [2003-10-25 19:28:13]
Regarding the post by fgilles on April 26th 2001, I suggest not to use levenshtein() function to test for over-uppercasing unless you've got plenty of time to waste in your host. ;) Anyhow, I think it's a useful feature, as I get really annoyed when reading whole messages in uppercase.
PHP's levenshtein() function can only handle up to 255 characters, which is not realistic for user input (only the first paragraph oh this post has 285 characters). If you choose to use a custom function able to handle more than 255 characters, efficiency is an important issue.
I use this function, specific for this case, but much faster:
function ucase_percent ($str) {
$str2 = strtolower ($str);
$l = strlen ($str);
$ucase = 0;
for ($i = 0; $i < $l; $i++) {
if ($str{$i} != $str2{$i}) {
$ucase++;
}
}
return $ucase / $l * 100.0;
}
I think 10% is enough for written English (maybe other languages like German, which use more capital letters, need more). With some sentencies in uppercase (everybody has the right to shout occasionally), 20% would be enough; so I use a threshold of 30%. When exceeded, I lowercase the whole message.
Hope you find it useful and it helps keeping the web free of ill-mannered people.
[#19] "inerte" is my hotmail.com username [2003-07-11 10:22:12]
I am using this function to avoid duplicate information on my client's database.
After retrieving a series of rows and assigning the results to an array values, I loop it with foreach comparing its levenshtein() with the user supplied string.
It helps to avoid people re-registering "John Smith", "Jon Smith" or "Jon Smit".
Of course, I can't block the operation if the user really wants to, but a suggestion is displayed along the lines of: "There's a similar client with this name.", followed by the list of the similar strings.
[#20] bisqwit at iki dot fi [2002-08-12 07:43:30]
[#21] [2002-04-11 18:57:56]
For spell checking applications, delay could be tolerable if you assume the typist got the first two or three chars of each word right. Then you'd only need to calc distances for a small segment of the dictionary. This is a compromise but one I think a lot of spell checkers make.
For an example of site search using this function look at the PHP manual search button on this page. It appears to be doing this for the PHP function list.
[#22] fgilles at free dot fr [2001-04-26 13:32:55]
Exempla of use for a forum: users can't post messages too much uppercased
<?php
if ((strlen($subject)>10) and ( ( levenshtein ($subject, strtolower ($subject) / strlen ($subject) ) > .3 ) ){
$subject = strtolower($subject);
}
?>
[#23] dschultz at protonic dot com [2000-08-10 09:01:14]
It's also useful if you want to make some sort of registration page and you want to make sure that people who register don't pick usernames that are very similar to their passwords.
[#24] ad1n at dc dot uba dot ar [2000-08-04 17:01:01]
One application of this is when you want to look for a similar match instead of an exact one. You can sort the results of checking the distances of a word to a dictionary and sort them to see which were the more similar ones. Of course it will be a quite resourse consuming task anyway.