---single[single_pat]---
This line has [OMIT]some words[/OMIT] omitted [OMIT]twice[/OMIT].
↓
This line has  omitted .

---nested[nested_pat]---
on [OMIT]this line, [OMIT]you have[/OMIT] to [/OMIT]count
↓
on count

---multi[multi_pat]---
this [OMIT] line
doesn't have [/OMIT]omissions
↓
this omissions

<?php

header('Content-Type: text/plain; charset=utf-8');

$show_borken = !!0;


### C-like preprocessor. ###
$pp_pat = <<<END
    ( ^ [#] \s* begin_omit \s* $ )
    ( (?: (?R) | . )*? )
    ( ^ [#] \s* end_omit \s* $ )
END;
echo preg_replace("@$pp_pat@msx", '', $myself);