permutation using regular expression in php

Question

Is there any way to get permutation of all words in a string using only regular expression in php.

For example:

for input like "How to remove pain in human knee"

I want output as:

"How To", "How","pain knee","remove knee","knee pain","in remove","human pain knee", etc.

Answer 1

Using regex will only serve to slow down the process.

I have written a function based on the powerSet() function posted by Yada @ https://stackoverflow.com/a/27968556/2943403)

Code: (Demo)

function permStacker($array){
    $stack=[[]];  // declare intitial empty element
    foreach($array as $element){
        foreach($stack as $combination){
            foreach(array_diff($array,$combination) as $left){
                $merge=array_merge($combination,[$left]);
                $stack[implode(' ',$merge)]=$merge;  // keys hold desired strings, values hold subarray of combinations for iterated referencing
            }
        }
    }
    unset($stack[0]); // remove initial empty element
    return array_keys($stack);  // return array of permutations as space delimited strings
}

$string='How to remove pain in human knee';
$permutations=permStacker(explode(' ',$string));
echo 'Total Permutations: ',sizeof($permutations),"\n";
var_export($permutations);

Output:

Total Permutations: 13699
array (
  0 => 'How',
  1 => 'to',
  2 => 'remove',
  3 => 'pain',
  4 => 'in',
  5 => 'human',
  6 => 'knee',
  7 => 'How to',
  8 => 'How remove',
  9 => 'How pain',
  10 => 'How in',
  11 => 'How human',
  12 => 'How knee',
  13 => 'to How',
  14 => 'to remove',
  15 => 'to pain',
  16 => 'to in',
  17 => 'to human',
  18 => 'to knee',
  19 => 'remove How',
  20 => 'remove to',
  21 => 'remove pain',
  22 => 'remove in',
  23 => 'remove human',
  24 => 'remove knee',
  25 => 'pain How',
  26 => 'pain to',
  27 => 'pain remove',
  28 => 'pain in',
  29 => 'pain human',
  30 => 'pain knee',
  31 => 'in How',
  32 => 'in to',
  33 => 'in remove',
  34 => 'in pain',
  35 => 'in human',
  36 => 'in knee',
  37 => 'human How',
  38 => 'human to',
  39 => 'human remove',
  40 => 'human pain',
  41 => 'human in',
  42 => 'human knee',
  43 => 'knee How',
  44 => 'knee to',
  45 => 'knee remove',
  46 => 'knee pain',
  47 => 'knee in',
  48 => 'knee human',
  49 => 'How to remove',
  50 => 'How to pain',
  51 => 'How to in',

Here is a post by bob at math.stackexchange which confirms that 13699 is the expected size of the returned array when given 7 words in the input string. Furthermore, bob's breakdown should serve as a warning about how the permutations ramp up rather fast -- be careful with your big strings.