Generating alpha-numeric string in PHP

Question

I have a PHP function that can generate alpha-numeric string with given length. The function works fine. But the generated string includes digits sometimes, and sometimes it does not include digits. I want that the string must consist of number and alpha every time. Need suggestion for that. Here is the code of the function:

public static function GenerateAlphaNumString($length=0) {
        $characterPool = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
        $randomString = '';

        for ($i = 0; $i < $length; $i++) {
            $randomString .= $characterPool[rand(0, strlen($characterPool) - 1)];
        }

        return $randomString;
    }

How many letters and how many digits? And you can't have both if $length < 2. — Kenney
– Kenney, Commented Jan 31, 2016 at 14:19
If it is supposed to be random, you can't expect it to always include digits ... Also, rand() is not really random. — Narf
– Narf, Commented Jan 31, 2016 at 14:20
No limit for digit. But the string should contain minimum one digit. Unlike now, because sometimes it generates string without any digit. — Muminur Rahman
– Muminur Rahman, Commented Jan 31, 2016 at 14:22

arkascha · Accepted Answer · 2016-01-31 14:34:20Z

4

A full example, offering flexibility:

<?php

class StringGenerator
{
  private $characterPool = [
    'numeric' => '0123456789',
    'alpha' => 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ'
  ];

  function generateRandomString($length, $pool) {
    $randomString = '';
    for ($i = 0; $i  < $length; $i++) {
      $randomString .= $this->characterPool[$pool][rand(0, strlen($this->characterPool[$pool]) - 1)];
    }
    return $randomString;
  }

  function generateAlphaNumString($length=0, $numeric=0) {
    return str_shuffle(
      $this->generateRandomString($numeric, 'numeric') .
      $this->generateRandomString($length-$numeric, 'alpha')
    );
  }
}

$generator = new StringGenerator;
for ($c=0; $c<=10; $c++) {
  var_dump($generator->generateAlphaNumString(10, 5));
}

The output is something like:

string(10) "J5F4FQX480"
string(10) "Z2g647pO9a"
string(10) "159Mf8GEL3"
string(10) "B5P57I42hN"
string(10) "Us97v5cg80"
string(10) "34VEHY825t"
string(10) "V6Ei5p06s1"
string(10) "YS5noD5127"
string(10) "4dJ4Y4G4q4"
string(10) "C8V7W3Q2G9"
string(10) "gu28g99H9d"

answered Jan 31, 2016 at 14:34

arkascha

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3 Comments

trincot Over a year ago

This fixes the number of generated digits (5 in this case), which makes it a bit less random than the original.

arkascha Over a year ago

@trincot That is indeed true, it is how I understood the OPs question, though I agree this can be seen in different ways... Nothing speaks against randomizing the number of numericals in the final call, though...

Mike Miller Over a year ago

A much better solution

trincot · Accepted Answer · 2016-01-31 14:54:28Z

3

You could do it like this:

// Add a quick exit:
if ($length <= 0) return '';

// Produce all characters except the last one: 
for ($i = 0; $i < $length-1; $i++) {
    $randomString .= $characterPool[rand(0, strlen($characterPool) - 1)];
}

// Check if string contains a digit, if not, reduce range 
// for generating last character so it can only be a digit:
$lastRange = preg_match("/\d/", $randomString) ? strlen($characterPool) : 10;
// choose random position, and insert last character in there:
$randomString = substr_replace($randomString, $characterPool[rand(0, $lastRange-1)], 
                               rand(0, $length-1), 0);

return $randomString;

This method will maintain the probability for each character's occurrence as close as possible to the original. It does not force digits to be grouped together either.

Obviously the right mix can only be generated when the given length is at least 2.

edited Jan 31, 2016 at 14:54

answered Jan 31, 2016 at 14:28

trincot

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4 Comments

Muminur Rahman Over a year ago

Thanks. It works great and clean. Thank you so much. But can I randomize the position of the number when I forced to enter number?

trincot Over a year ago

Yes, that is possible. I improved the code to insert last chosen character at random position.

Muminur Rahman Over a year ago

Thanks for the changes and comments in the code. The comments helped me so much as I am a newbie. :)

trincot Over a year ago

Even for non-newbie, comments are an essential part of good code. Glad you can appreciate it!

Robin De Bruin · Accepted Answer · 2016-01-31 14:21:44Z

3

This should do the trick

$characterPool = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
$randomString = '';
$numberscount = rand(1,$length-1);
for ($i = 0; $i < $numberscount; $i++) {
    $randomString .= rand(0,9);
}
for ($i = 0; $i < ($length - $numberscount); $i++) {
    $randomString .= $characterPool[rand(0, strlen($characterPool) - 1)];
}

it always prints a character when $length = 1 though.

answered Jan 31, 2016 at 14:21

Robin De Bruin

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1 Comment

trincot Over a year ago

This tends to concentrate digits at the start of the string which makes it a little bit less random.

Mike Miller · Accepted Answer · 2016-01-31 14:41:32Z

2

Add the following before you return the string

If(!preg_match( '/([0-9]+)/', $randomString) {
     return self::GenerateAlphaNumString($length);
}

If there are no numbers in your string just recall same function until there is.

edited Jan 31, 2016 at 14:41

answered Jan 31, 2016 at 14:28

Mike Miller

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3 Comments

arkascha Over a year ago

I'd call this somewhat inefficient. Kind of the famous "random sort algorithm"...

trincot Over a year ago

Nice! You should add return before that recursive call though.

Mike Miller Over a year ago

Good spot. Edited my answer. If efficiency is a major concern this may not be the best solution but probability of having to iterate second time is reasonably small so for most cases I reckon this is ok

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