<?php
 
// your code goes here
$regex = '{
    (?(DEFINE)
       (?<number>    (?: -? (?: (?= [1-9]|0(?!\d) ) \d+ (?:\.\d+)? ) | \.\d+ ) )
       (?<variable>  (?: \bM\b ) )
       (?<clause>    (?: 
                         (?&variable) \s* [<>=] \s* (?&number) |
                         (?&variable) \s+ between \s+ (?&number) \s+ and \s+ (?&number)
                     ))
    )
    (?&clause)
}x';
 
$str = '(a > 3) and ( b + c = 4 and M < 4) and (d + e = 6 and M between 4 and 6) and z>10';
if (preg_match_all($regex, $str, $matches)) {
    foreach ($matches[0] as $m) {
      echo $m. PHP_EOL;
    }
}

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