[imp] text part not displayed

[Per olof Ljungmark]

Only the download link is visible on text only messages. HTML is
displayed OK.

Latest stable Horde4/IMP5
PHP 5.3.7

logg:

WARN: [imp] PHP ERROR: preg_replace_callback(): Compilation failed:
POSIX collating elements are not supported at offset 255 [pid 46962 on
line 85 of "/usr/local/share/pear/Horde/Text/Filter.php"]

prefs

$mime_drivers = array(
    /* Plain text viewer. */
    'plain' => array(
        'inline' => true,
        'handles' => array(
            'application/pgp',
            'text/plain',
            'text/rfc822-headers'
        ),

Suggestions anyone?