Print RegEx matches using SED in bash

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http://stackoverflow.com – I have an XML file, the file is made up of one line. What I am trying to do is extract the "finalNumber" attribute value from the file via Putty. Rather than having to download a copy and search using notepad++. I've built up a regular expression that I've tested on an On-line Tool, and tried using it within a sed command to duplicate grep functionality. The command runs but doesn't return anything. RegEx: (?<=finalNumber=")(.*?)(?=") sed Command (returns nothing, expected 28, see file extract): sed -n '/(?<=finalNumber=")(.*?)(?=")/p' file.xml File Extract: ...argo:finalizedD (HowTos)