You may use this regex in php using preg_match_all:

preg_match_all('/(?<![a-zA-Z])[a-zA-Z]\K\d+/', $string, $matches);

Resulting in array $matches[0] to return all the matches.

RegEx Demo

RegEx Details:

• (?<![a-zA-Z]): Make sure we don't have a letter before current position
• [a-zA-Z]: Match a letter
• \K: Reset match info
• \d+: Match 1+ digits

Method

1. You need to strip out the variable patterns; you can use preg_match_all to do this

   preg_match_all("/(\[\w+\]|\([\w|]+\))/", '[ct]hun(k|der)(s|ed|ing)?', $matches);
   
   /* Regex:
   
   /(\[\w+\]|\([\w|]+\))/
   /                       : Pattern delimiter
    (                      : Start of capture group
     \[\w+\]               : Character class pattern
            |              : OR operator
             \([\w|]+\)    : Capture group pattern
                       )   : End of capture group
                        /  : Pattern delimiter
   
   */
   

2. You can then expand the capture groups to letters or words (depending on type)

   $array = str_split($cleanString, 1); // For a character class
   $array = explode("|", $cleanString); // For a capture group
   

3. Recursively work your way through each $array

Code

function printMatches($pattern, $array, $matchPattern)
{
    $currentArray = array_shift($array);

    foreach ($currentArray as $option) {
        $patternModified = preg_replace($matchPattern, $option, $pattern, 1);
        if (!count($array)) {
            echo $patternModified, PHP_EOL;
        } else {
            printMatches($patternModified, $array, $matchPattern);
        }
    }
}

function prepOptions($matches)
{
    foreach ($matches as $match) {
        $cleanString = preg_replace("/[\[\]\(\)\?]/", "", $match);
        
        if ($match[0] === "[") {
            $array = str_split($cleanString, 1);
        } elseif ($match[0] === "(") {
            $array = explode("|", $cleanString);
        }
        if ($match[-1] === "?") {
            $array[] = "";
        }
        $possibilites[] = $array;
    }
    return $possibilites;
}

$regex        = '[ct]hun(k|der)(s|ed|ing)?';
$matchPattern = "/(\[\w+\]|\([\w|]+\))\??/";

preg_match_all($matchPattern, $regex, $matches);

printMatches(
    $regex,
    prepOptions($matches[0]),
    $matchPattern
);

Additional functionality

Expanding nested groups

In use you would put this before the "preg_match_all".

$regex        = 'This happen(s|ed) to (be(come)?|hav(e|ing)) test case 1?';

echo preg_replace_callback("/(\(|\|)(\w+)(?:\(([\w\|]+)\)\??)/", function($array){
    $output = explode("|", $array[3]);
    if ($array[0][-1] === "?") {
        $output[] = "";
    }
    foreach ($output as &$option) {
        $option = $array[2] . $option;
    }
    return $array[1] . implode("|", $output);
}, $regex), PHP_EOL;

Output:

This happen(s|ed) to (become|be|have|having) test case 1?

Matching single letters

The bones of this would be to update the regex:

$matchPattern = "/(?:(\[\w+\]|\([\w|]+\))\??|(\w\?))/";

and add an else to the prepOptions function:

} else {
    $array = [$cleanString];
}

Full working example

function printMatches($pattern, $array, $matchPattern)
{
    $currentArray = array_shift($array);

    foreach ($currentArray as $option) {
        $patternModified = preg_replace($matchPattern, $option, $pattern, 1);
        if (!count($array)) {
            echo $patternModified, PHP_EOL;
        } else {
            printMatches($patternModified, $array, $matchPattern);
        }
    }
}

function prepOptions($matches)
{
    foreach ($matches as $match) {
        $cleanString = preg_replace("/[\[\]\(\)\?]/", "", $match);
        
        if ($match[0] === "[") {
            $array = str_split($cleanString, 1);
        } elseif ($match[0] === "(") {
            $array = explode("|", $cleanString);
        } else {
            $array = [$cleanString];
        }
        if ($match[-1] === "?") {
            $array[] = "";
        }
        $possibilites[] = $array;
    }
    return $possibilites;
}

$regex        = 'This happen(s|ed) to (be(come)?|hav(e|ing)) test case 1?';
$matchPattern = "/(?:(\[\w+\]|\([\w|]+\))\??|(\w\?))/";

$regex = preg_replace_callback("/(\(|\|)(\w+)(?:\(([\w\|]+)\)\??)/", function($array){
    $output = explode("|", $array[3]);
    if ($array[0][-1] === "?") {
        $output[] = "";
    }
    foreach ($output as &$option) {
        $option = $array[2] . $option;
    }
    return $array[1] . implode("|", $output);
}, $regex);


preg_match_all($matchPattern, $regex, $matches);

printMatches(
    $regex,
    prepOptions($matches[0]),
    $matchPattern
);

Output:

This happens to become test case 1
This happens to become test case 
This happens to be test case 1
This happens to be test case 
This happens to have test case 1
This happens to have test case 
This happens to having test case 1
This happens to having test case 
This happened to become test case 1
This happened to become test case 
This happened to be test case 1
This happened to be test case 
This happened to have test case 1
This happened to have test case 
This happened to having test case 1
This happened to having test case