Question

I want to find a given sub string that exist or match in a big string.

-> a given sub string can contain \n \r \r

I try

if(preg_match("`".$sub_str."(.*)`im", $str, $matches)){

     //DO something when it true

}

for that case I did not know what can be the problems with any given sub string?

Answer 1

Modifier i is case insensitive flag.

Modifier m is multiline flag.

This is, from what I can tell at a very quick glance, a terrible way of doing:

$pos = stripos($find_it, $str);
if ($pos !== false) {
    // $find_it exists within $str, $matches contains string after
    $matches = substr($str, $pos);
}

The regular expression is simply finding (in a case-insensitive, multiline-aware manner) a ($find_it) within another string ($str) and placing everything after that into a capturing group (within $matches). The above is a loose match of the code, just faster.

---

Answering your question directly, preg_match would return false when $find_it did not exist within $str.