Text input problems can be setup to accept open-ended responses using regular expressions in the advanced editor.
To format your text input problem to accept any response, assign the answer attribute as follows:
Here is an example open input text with a regular expression, note the regexp within the type attribute. In this example, the size of the text line has been set to 20 characters as the default. You are free to add more characters if you anticipate longer responses.
<stringresponse answer=".*." type="ci regexp"> <textline size="20"/> </stringresponse>
Note: The open-ended text input problem allows for any text response, including special characters, and can not prevent non-meaningful text from being submitted.