<?php

$exp = '/M\D*[\d\.]+\s*[a-zA-Z]*\s*[\d\.]+/';
$str = '(a >3) and ( b + c = 4 and M < 45) and (d + e = 6 and M between 40 and 60) and z>10';
    
preg_match_all($exp, $str, $match);
$palavras_encontradas = $match[0];

var_dump($palavras_encontradas);