Removing Strings from both left & right in dynamic changing string in PHP

Question

I have an array that contains more than 100 keys changing dynamically.

([chap1e1], [chap1m1], [chap1h1], ..., [chap100e1], [chap100m1], [chap100h1], ...)

I used the following to remove string from left & right to obtain the dynamic number changing (i.e. 1 - 100 - ...). But here, at the right I need to remove is the last 2 characters exactly.

foreach ($_POST as $k => $v) {    
    $unit = substr(substr($k, 4), 1, -2);
    echo $unit;
}

But the logic seems to be working until, the number is (one digit).

Is there any alternative way of doing this dynamic trim of string from both left & right? Please suggest any idea. Thank You!

SOLUTION using substr()

$unit = substr(substr($k, 4), 0, -2)

Answer 1

You can try a regular expression match, eg

if (preg_match('/^[A-Za-z]{4}(\d+)/', $k, $matches)) {
    echo $matches[1];
}

This assumes that the alpha string on the left is only ever 4 characters (ie {4} in the pattern)