Count non-static text values

Hello,

I have a report which uses the 'text filter' visual to search for specific text within a field, and filter the report accordingly.  This all works very effectively. The text filter visual filters a matrix from which they can see the responses they need. 

I have received a request for the user to be able to search for a piece of text, and then have a visual which counts the number of times the text they have searched appears in the relevant field. I have struggled to figure out an effective way to do this, has anyone came across a similar scenario? I know how to do this with static values that I could add in a measure, but not for ones that a user defines. Any tips would be appreciated! 

Thank you