similar_text

(PHP 4, PHP 5, PHP 7, PHP 8)

similar_textBerechnet die Ähnlichkeit zweier Zeichenketten

Beschreibung

similar_text(string $string1, string $string2, float &$percent = null): int

Die Berechnung der Ähnlichkeit zweier Strings erfolgt nach der in Programming Classics: Implementing the World's Best Algorithms by Oliver (ISBN 0-131-00413-1) beschriebenen Methode. Beachten Sie, dass diese Implementierung keinen Stack verwendet wie in Olivers Pseude-Code, dafür aber rekursive Aufrufe, die die Performance erhöhen können (aber nicht müssen). Beachten Sie auch, dass die Komplexität dieses Algorithmus O(N**3) beträgt, wobei N die Länge der längsten Zeichenkette ist.

Parameter-Liste

string1

Die erste Zeichenkette.

string2

Die zweite Zeichenkette.

Hinweis:

Das Vertauschen der Parameter first und second kann ein anderes Ergebnis zur Folge haben; siehe das Beispiel weiter unten.

percent

Falls Sie als Referenz ein drittes Argument angeben, wird similar_text() die Ähnlichkeit als Prozentwert errechnen. Dazu wird das Ergebnis von similar_text() durch den Durchschnitt der Längen der angegebenen Zeichenketten dividiert und mit 100 multipliziert.

Rückgabewerte

Gibt die Anzahl übereinstimmender Zeichen in beiden Zeichenketten zurück.

Die Anzahl übereinstimmender Zeichen wird berechnet, indem die längste erste gemeinsame Teilzeichenkette gesucht wird und dies dann rekursiv für die Prä- und Suffixe wiederholt wird. Die Längen aller gefundenen gemeinsamen Teilzeichenketten werden aufsummiert.

Beispiele

Beispiel #1 Beispiel für die Vertauschung der Argumente von similar_text()

Dieses Beispiel verdeutlicht, dass das Vertauschen des first und second Arguments ein anderes Ergebnis zur Folge haben kann.

<?php
$sim
= similar_text('bafoobar', 'barfoo', $perc);
echo
"Ähnlichkeit: $sim ($perc %)\n";
$sim = similar_text('barfoo', 'bafoobar', $perc);
echo
"Ähnlichkeit: $sim ($perc %)\n";

Das oben gezeigte Beispiel erzeugt eine ähnliche Ausgabe wie:

Ähnlichkeit: 5 (71.428571428571 %)
Ähnlichkeit: 3 (42.857142857143 %)

Siehe auch

  • levenshtein() - Berechnet die Levenshtein-Distanz zwischen zwei Strings
  • soundex() - Berechnet die Laut-Ähnlichkeit eines Strings

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User Contributed Notes 11 notes

up
108
SPAM HATER
12 years ago
Hey there,

Be aware when using this function, that the order of passing the strings is very important if you want to calculate the percentage of similarity, in fact, altering the variables will give a very different result, example :

<?php
$var_1
= 'PHP IS GREAT';
$var_2 = 'WITH MYSQL';

similar_text($var_1, $var_2, $percent);

echo
$percent;
// 27.272727272727

similar_text($var_2, $var_1, $percent);

echo
$percent;
// 18.181818181818
?>
up
93
daniel dot karbach at localhorst dot tv
13 years ago
Please note that this function calculates a similarity of 0 (zero) for two empty strings.

<?php
similar_text
("", "", $sim);
echo
$sim; // "0"
?>
up
22
I_HATE_SPAMMER- PAZ!
10 years ago
Actually similar_text() is not bad...
it works good. But before processing i think is a good way to make a little mod like this

$var_1 = strtoupper("doggy");
$var_2 = strtoupper("Dog");

similar_text($var_1, $var_2, $percent);

echo $percent; // output is 75 but without strtoupper output is 50
up
19
ryan at derokorian dot com
10 years ago
Note that this function is case sensitive:

<?php

$var1
= 'Hello';
$var2 = 'Hello';
$var3 = 'hello';

echo
similar_text($var1, $var2);  // 5
echo similar_text($var1, $var3);  // 4
up
15
vasyl at vasyltech dot com
8 years ago
Recursive algorithm usually is very elegant one. I found a way to get better precision without the recursion. Imagine two different (or same) length ribbons with letters on each. You simply shifting one ribbon to left till it matches the letter the first.

<?php

function similarity($str1, $str2) {
   
$len1 = strlen($str1);
   
$len2 = strlen($str2);
   
   
$max = max($len1, $len2);
   
$similarity = $i = $j = 0;
   
    while ((
$i < $len1) && isset($str2[$j])) {
        if (
$str1[$i] == $str2[$j]) {
           
$similarity++;
           
$i++;
           
$j++;
        } elseif (
$len1 < $len2) {
           
$len1++;
           
$j++;
        } elseif (
$len1 > $len2) {
           
$i++;
           
$len1--;
        } else {
           
$i++;
           
$j++;
        }
    }

    return
round($similarity / $max, 2);
}

$str1 = '12345678901234567890';
$str2 = '12345678991234567890';

echo
'Similarity: ' . (similarity($str1, $str2) * 100) . '%';
?>
up
1
pablo dot pazos at cabolabs dot com
3 years ago
To calculate the percentage of similarity between two strings without depending on the order of the parameters and be case insensitive, I use this function based on levenshtein's distance:

<?php

 
// string similarity calculated using levenshtein
 
static function similarity($a, $b)
  {
    return
1 - (levenshtein(strtoupper($a), strtoupper($b)) / max(strlen($a), strlen($b)));
  }

?>

This will always return a number between 0 and 1, representing the percentage, for instance 0.8 represents 80% similar strings.

If you want this to be case-sensitive, just remove the strtoupper() functions.
up
9
daniel at reflexionsdesign dot com
23 years ago
If performance is an issue, you may wish to use the levenshtein() function instead, which has a considerably better complexity of O(str1 * str2).
up
11
Paul
17 years ago
The speed issues for similar_text seem to be only an issue for long sections of text (>20000 chars).

I found a huge performance improvement in my application by just testing if the string to be tested was less than 20000 chars before calling similar_text.

20000+ took 3-5 secs to process, anything else (10000 and below) took a fraction of a second.
Fortunately for me, there was only a handful of instances with >20000 chars which I couldn't get a comparison % for.
up
12
julius at infoguiden dot no
21 years ago
If you have reserved names in a database that you don't want others to use, i find this to work pretty good.
I added strtoupper to the variables to validate typing only. Taking case into consideration will decrease similarity.

<?php
$query
= mysql_query("select * from $table") or die("Query failed");

while (
$row = mysql_fetch_array($query)) {
     
similar_text(strtoupper($_POST['name']), strtoupper($row['reserved']), $similarity_pst);
      if (
number_format($similarity_pst, 0) > 90){
       
$too_similar = $row['reserved'];
        print
"The name you entered is too similar the reserved name &quot;".$row['reserved']."&quot;";
        break;
       }
    }
?>
up
3
georgesk at hotmail dot com
22 years ago
Well, as mentioned above the speed is O(N^3), i've done a longest common subsequence way that is O(m.n) where m and n are the length of str1 and str2, the result is a percentage and it seems to be exactly the same as similar_text percentage but with better performance... here's the 3 functions i'm using..

<?php
function LCS_Length($s1, $s2)
{
 
$m = strlen($s1);
 
$n = strlen($s2);

 
//this table will be used to compute the LCS-Length, only 128 chars per string are considered
 
$LCS_Length_Table = array(array(128),array(128));
 
 
 
//reset the 2 cols in the table
 
for($i=1; $i < $m; $i++) $LCS_Length_Table[$i][0]=0;
  for(
$j=0; $j < $n; $j++) $LCS_Length_Table[0][$j]=0;

  for (
$i=1; $i <= $m; $i++) {
    for (
$j=1; $j <= $n; $j++) {
      if (
$s1[$i-1]==$s2[$j-1])
       
$LCS_Length_Table[$i][$j] = $LCS_Length_Table[$i-1][$j-1] + 1;
      else if (
$LCS_Length_Table[$i-1][$j] >= $LCS_Length_Table[$i][$j-1])
       
$LCS_Length_Table[$i][$j] = $LCS_Length_Table[$i-1][$j];
      else
       
$LCS_Length_Table[$i][$j] = $LCS_Length_Table[$i][$j-1];
    }
  }
  return
$LCS_Length_Table[$m][$n];
}

function
str_lcsfix($s)
{
 
$s = str_replace(" ","",$s);
 
$s = ereg_replace("[��������]","e", $s);
 
$s = ereg_replace("[������������]","a", $s);
 
$s = ereg_replace("[��������]","i", $s);
 
$s = ereg_replace("[���������]","o", $s);
 
$s = ereg_replace("[��������]","u", $s);
 
$s = ereg_replace("[�]","c", $s);
  return
$s;
}
 
function
get_lcs($s1, $s2)
{
 
//ok, now replace all spaces with nothing
 
$s1 = strtolower(str_lcsfix($s1));
 
$s2 = strtolower(str_lcsfix($s2));
 
 
$lcs = LCS_Length($s1,$s2); //longest common sub sequence

 
$ms = (strlen($s1) + strlen($s2)) / 2;

  return ((
$lcs*100)/$ms);
}
?>

you can skip calling str_lcsfix if you don't worry about accentuated characters and things like that or you can add up to it or modify it for faster performance, i think ereg is not the fastest way?
hope this helps.
Georges
up
-1
Anonymous
4 years ago
$result = similar_text ('ab', 'a', $percent);

> $percent: 66.666666666666671
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