Ponctuation is replaced when set to "No action"

I wish to keep files with their extension like .doc, .xls, ...
If I set Period to "No action", it is still replaced by the default separator, so file name end by -doc, -xls, ...

The same for any punctuation setting: "No action" is equivalent to "Replace".

This happens here, at the "string to remove" stage:

  // Get rid of words that are on the ignore list
  if ($cache['ignore_words_regex']) {
    $words_removed = $cache['ignore_words_callback']($cache['ignore_words_regex'], '', $output);
    dpm($cache, "cache");
    dpm($words_removed, "words_removed");
    if (drupal_strlen(trim($words_removed)) > 0) {
      $output = $words_removed;
    }
  }

My list does not contain any punctuation though.
The $cache['ignore_words_regex'] contains the following:

\ba\b|\ban\b|\bas\b|\bat\b|\bbefore\b|\bbut\b|\bby\b|\bfor\b|\bfrom\b|\bis\b|\bin\b|\binto\b|\blike\b|\bof\b|\boff\b|\bon\b|\bonto\b|\bper\b|\bsince\b|\bthan\b|\bthe\b|\bthis\b|\bthat\b|\bto\b|\bup\b|\bvia\b|\bwith\b|\bà\b|\bainsi\b|\balors\b|\baprès\b|\baussi\b|\bavant\b|\bavec\b|\bcar\b|\bcertes\b|\bchez\b|\bcomme\b|\bd\b|\bdans\b|\bde\b|\bdehors\b|\bdepuis\b|\bderrière\b|\bdes\b|\bdonc\b|\ben\b|\bensuite\b|\benvers\b|\bet\b|\betc\b|\bici\b|\bjusqu\b|\bjusque\b|\bl\b|\bla\b|\ble\b|\bles\b|\blà\b|\bmais\b|\bmême\b|\bni\b|\bor\b|\boù\b|\bpar\b|\bpendant\b|\bpour\b|\bprès\b|\bsans\b|\bsauf\b|\bselon\b|\bsous\b|\bsur\b|\btout\b|\bun\b|\bune\b|\bvers\b|\bvu\b

the called back function mb_eregi_replace() seems to replace all punctuation that subsist at this stage.

Thanks for help