Your issue is a limitation of PHP REGEX functions. Your string is too long for it. I recommend using a CB Query Field to query for the description and cut it short directly in the query. Then substitute in the query result. Example as follows.
Type: Query
Name: cb_description_short
Query:
Code:
SELECT SUBSTR( `cb_description`, 0, 50 ) FROM `#__comprofiler` WHERE `id` = '[user_id]'
Code:
Code:
<meta name="description" content="[cb_description_short]" />