https://community.blueprism.com/t5/Product-Forum/Collection-Filter-Use-Mid/m-p/76446#M28945 Your expression should not have quotation marks around it.    From your description you should have something more like this as your expression in a dedcision stage: Mid([Producer Code], 2, 6)=""ABCDEF""   I strongly recommend that people use the expression builder functionality in the Decision/Calculation stage to build their expressions if they are not sure of Blue Prism syntax. Fri, 27 Apr 2018 13:54:00 GMT Denis__Dennehy 2018-04-27T13:54:00Z https://community.blueprism.com/t5/Product-Forum/Collection-Filter-Use-Mid/m-p/76445#M28944 I have a collection with field (text) named [Producer Code], now i want to filter this collection where the 2nd to 7th characters are "ABCDEF" in [Producer Code], how to write this filter. "ABCDEF" may appear at anywhere,but they are not what i want except for the 2nd to 7th.   I entered a filter as below, but system indicate : The expression contains undefined function call Mid(). "Mid([Producer Code], 2, 6)='"&[ProducerCode]&"'" Tue, 24 Apr 2018 13:59:00 GMT https://community.blueprism.com/t5/Product-Forum/Collection-Filter-Use-Mid/m-p/76445#M28944 EricLi 2018-04-24T13:59:00Z https://community.blueprism.com/t5/Product-Forum/Collection-Filter-Use-Mid/m-p/76446#M28945 Your expression should not have quotation marks around it.    From your description you should have something more like this as your expression in a dedcision stage: Mid([Producer Code], 2, 6)=""ABCDEF""   I strongly recommend that people use the expression builder functionality in the Decision/Calculation stage to build their expressions if they are not sure of Blue Prism syntax. Fri, 27 Apr 2018 13:54:00 GMT https://community.blueprism.com/t5/Product-Forum/Collection-Filter-Use-Mid/m-p/76446#M28945 Denis__Dennehy 2018-04-27T13:54:00Z