First thing to note, you should be using storeText, not store. Store will only record what you put in the target field, it won't look for the locator on the page. Also, the way you've done your regex ([0-9]{6}) won't give you what you'd need. That would look for a digit from 0-9 followed by 6 more digits.
I've recently had to do pretty much the same thing, the way I did it is separated this out into 2 commands, rather than trying to process it all in one go. so first command, store the full thing, second command, Regex to pull out the 6 digits. Like below
<tr>
<td>storeText</td>
<td>//table[@id='sms_table']/tbody/tr/td/pre</td>
<td>Text</td>
</tr>
<tr>
<td>storeEval</td>
<td>storedVars['Text'].match(/\d{6}/)</td>
<td>digits</td>
</tr>
matches()function which is only available in XPath 2.0. Selenium-IDE uses XPath 1.0.